What is the Advantage and Disadvantage of Arsenic Removal Furnace Manufacturing

Abstract

Arsenic is a toxic metalloid which is widely distributed in nature. It is normally present as arsenate under oxic conditions while arsenite is predominant under reducing condition. The major discharges of arsenic in the environment are mainly due to natural sources such as aquifers and anthropogenic sources. It is known that arsenite salts are more toxic than arsenate as it binds with vicinal thiols in pyruvate dehydrogenase while arsenate inhibits the oxidative phosphorylation process. The common mechanisms for arsenic detoxification are uptaken by phosphate transporters, aquaglyceroporins, and active extrusion system and reduced by arsenate reductases via dissimilatory reduction mechanism. Some species of autotrophic and heterotrophic microorganisms use arsenic oxyanions for their regeneration of energy. Certain species of microorganisms are able to use arsenate as their nutrient in respiratory process. Detoxification operons are a common form of arsenic resistance in microorganisms. Hence, the use of bioremediation could be an effective and economic way to reduce this pollutant from the environment.

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1. Introduction

Arsenic is one of the toxic metalloids that exists in more than 200 different mineral forms, where 60% of them are normally arsenates; 20% are sulphosalts and sulphides; and the remaining 20% are arsenite, oxides, arsenide, silicates, and elemental arsenic [1, 2]. The intrusion of orogenesis and granitic magma have resulted in the formation of arsenopyrite [1]. Arsenic was first discovered by Albertus Magnus in the year [3]. Under natural condition, arsenic normally cycled at the earth surface where the breakdown of rocks has converted arsenic sulfides into arsenic trioxide [2, 4]. Furthermore, arsenic is known to have multiple oxidation states where they are present in either organic or inorganic compounds in an aquatic environment [5, 6]. Both Zobrist et al. [7] and Root et al. [8] indicated that the mobility of arsenic inorganic compound in contaminated aquatic and sediment environment is controlled by redox processes, precipitation, sorption, and dissolution processes. It is known that ferric iron phase plays an important role for the sorption of dissolved arsenate in oxic groundwater [8]. Meanwhile, the reduction of arsenate into arsenite in the transition from aerobic to anoxic pore waters is often mediated by microbial activity, which includes detoxification and metabolic mechanisms [8]. In another study, Saalfield and Bostick [9] proposed that the presence of calcium and bicarbonate from the byproducts of biological processes in the aquifers will enhance the release of arsenic and the correlations between calcium and bicarbonate with arsenic were then observed.

Arsenic usually exists in four oxidation states: As&#;3 (arsine), As° (arsenic), As+3 (arsenite), and As+5 (arsenate) [4, 10]. In soil environment, arsenic is generally present in two oxidation states which are As+3 (arsenite) and As+5 (arsenate) and normally present as a mixture of As+3 (arsenite) and As+5 (arsenate) in air [2]. Of the two oxidation states, arsenate is the main species associated with soil arsenic contaminations, and it is often written as AsO4 3&#; which is very similar to phosphate [11, 12]. Arsenate could act as a potential oxidative phosphorylation inhibitor. This is a cause for concern since oxidation phosphorylation is the main key reaction of energy metabolism in humans and metazoans [4]. Arsenite has been reported as the most toxic and soluble form of arsenic when compared to arsenate, and it can bind with reactive sulfur atoms present in many enzymes, including enzymes which are involved in respiration [4, 13]. Furthermore, it is known that soluble inorganic arsenic is often more toxic than the organic form [2]. Unlike arsenate and arsenite, arsine is often available as highly toxic gases such as (CH3)3 and H3As and often present at low concentration in the environment [4].

Meanwhile, the average concentration of arsenic in fresh water is around 0.4&#;μg/L and could reach 2.6&#;μg/L in seawater [13]. However, the thermal activity in some places has caused high level of arsenic in waters with the concentration of arsenic in geothermal water in Japan ranging from 1.8 to 6.4&#;mg/L whereas the concentration of arsenic in New Zealand water could reach up to 8.5&#;mg/L [2, 29, 30]. In extreme cases, analysis from well drinking water in Jessore, Bangladesh, revealed that the levels of arsenic could reach up until 225&#;mg/L [31]. On the other hand, the concentration of arsenic in plants is solely depending on the amount of arsenic that the plant is being exposed to where the concentration of arsenic could range from less than 0.01&#;μg/g (dried weight) in the uncontaminated area to around 5&#;μg/g (dried weight) in the contaminated area [2]. Unlike plant, the concentration of arsenic in marine organisms and mammals has a wide range of variations ranging from 0.005 to 0.3&#;mg/kg in some crustaceans and molluscs, 0.54&#;μg/g in fish, over 100&#;μg/g in some shellfish, and less than 0.3&#;μg/g in humans and domestic animals [2]. Presence of humic acid in the shallow subsurface could affect the mobility of arsenic since humic acid could interact with aqueous arsenic for the formation of stable colloidal complexes that might play a prominent role in the enhancement of arsenic mobility. Furthermore, the combination of humic acid together with ferric hydroxide surface will lead to the formation of stable complexes that would compete with arsenic for its adsorption sites [32].

2. Usage of Arsenic

The first usage of arsenic in medicine could be dated around years ago where it was mainly consumed for the improvement of breathing problems as well as to give freshness, beauty, and plumpness figures in women [2]. Arsenic in the form of arsenical salvarsan (arsenic containing drug) was the initial antimicrobial agent used in the treatment of infectious diseases such as syphilis and sleeping sickness in [3]. This drug was specifically developed by Sahachiro Hata under the guidance of Paul Ehrlich in where they named the drug as arsphenamine no. 606 [33]. Meanwhile, arsenic in the form of arsenic trioxide (As2O3) is one of the most common forms of arsenic, which is often used in manufacturing and agriculture industry and for medical purposes such as in the treatment of acute promyelocytic leukemia [34]. Arsenic trioxide is also proven to be useful in criminal homicides due to its characteristic, which is tasteless, colorless, highly toxic, and soluble in water [2, 4]. The high usage of arsenic trioxide in suicide cases had made it to be often referred as the &#;inheritance powder&#; in the 18th century [4].

During the s, arsenic was mainly used in agriculture industry in the form of insecticide's component in order to get rid of the insects [2, 13, 35]. Arsenic was also used as cotton desiccants and wood preservatives in United States [2]. The usage of arsenic as the cotton desiccant was introduced around year and was widely used due to its effectiveness and affordable price [36]. Besides that, arsenic was also being used in ceramic and glass industry, pharmaceutical industry, and food additives as well as pigments in paint [13, 34]. Meanwhile, arsenic in the form of 4-aminoben-zenearsenic acid (p-arsenilic acid, p-ASA) has been used as animals food additive for feeding of boiler chickens [37].

3. Toxicity of Arsenic

It has been noticed that the extensive usage of arsenic in the industrial and agrochemical applications is of few causes of groundwater and sediment arsenic contamination in the environment [6, 38] in which effects are much smaller compared to the natural causes [39]. The presence of arsenic in soil and water has become an increasing problem in many countries around the world, including Bangladesh, India, Chile, and Taiwan [2, 40, 41], and natural geological source is one of the main causes of contamination [34]. Consumption of drinking water that has been contaminated by hazardous level of arsenic will lead to a wide range of diseases such as arsenic dermatosis, lung cancer, liver cancer, uterus cancer, skin cancer and occurrence of skin, and bladder and hepatocellular carcinoma that will result in slow and painful death [1, 2, 41&#;43]. In Southwestern Taiwan, the human consumption of artesian well waters which contains high concentration of arsenic has also led to Blackfoot disease, which is an endemic peripheral vascular disease in that area [40]. In China, up to the year , 19 provinces had been found to have As concentration in drinking water exceeding the standard level (0.05&#;mg/L). Inner Mongolia, Xinjiang, and Shanxi Provinces are historical well-known &#;hotspots&#; of geogenic As-contaminated drinking water [44].

Deltaic plain contaminated groundwater of Ganges-Meghna-Brahmaputra rivers in Bangladesh and West Bengal had resulted in an alarming environmental problem as this water is often consumed by people who live in that area [6, 45]. The presence of aqueous arsenic is mainly due to rock weathering as well as sediment deposition and downstream transport of rich mineral arsenic that was originally present in Himalayas [4]. Massive constructions of wells which are meant to supply an improved quality of water with waterborne pathogens free to the people living in this area had created another problem as the ground water in that area was arsenic contaminated [4]. In Nepal, arsenic (As) contamination was a major issue in water supply drinking systems especially in high density population such as Terai districts. The local inhabitants still use hand tube and dug wells (with hand held pumps that are bored at shallow to medium depth) for their daily water requirements [46]. The results of the analysis on 25,058 samples tested in 20 districts, published in the report of arsenic in Nepal, demonstrated that 23% of the samples were containing 10&#;50&#;μg/L of As, and 8% of the samples were containing more than 50&#;μg/L of As. Recent status from over 737,009 samples tested has shown that 7.9% and 2.3% were contaminated by 10&#;50&#;μg/L and >50&#;μg/L of As, respectively [46]. Other places reporting the ground water arsenic contamination include south West Coast of Taiwan; Antofagasta in Chile; six areas of Region Lagunera located in the central part of North Mexico; Monte Quemado; Cordoba province in Argentina; Millard County in Utah, United States; Nova Scotia in Canada; and Inner Mongolia, Qinghai, Jilin, Shanxi, Xinjiang Uygur A.R., Ningxia, Liaoning, and Henan provinces in China [2].

Accidental ingestion of pesticides or insecticides containing arsenic will also result in an acute arsenic poisoning which sometimes could lead to mortality when 100&#;mg to 300&#;mg of doses were being consumed [1]. The symptoms of acute arsenic poisoning are vomiting, abdominal pain, diarrhea, and cramping, which will then cause renal failure, haematological abnormalities such as leukemia and anemia, pulmonary oedema, and respiratory failure, and it could further lead to shock, coma, and death [1, 2, 34]. In another study, Lai et al. [47] reported that the consumption of water contaminated with arsenic will increase the risk of diabetes mellitus by twofold. In US, prevalence of diabetes increased among people having urine arsenic concentrations in more than 20% of the general population [48]. Arsenic contamination from industrial sources has also led to skin manifestation of chronic arsenic poisoning, which affected 19.9% of the human populations living in Ron Phibun, Thailand [2]. On the other hand, arsenic poisoning caused by ingestion of food (especially seafood product) and beverages contaminated by arsenic has been reported in Japan, England, Germany, and China [2]. In Campinas, Brazil, 116 samples of seafood (used for sashimi making) from Japanese restaurants have been evaluated for the presence of As [49]. Several samples were found with percentage above the maximum limit permitted by European regulations including 90% tuna, 48% salmon, 31% mullet, and 100% octopus. It was concluded that the octopus was the sashimi which most contributed to arsenic. In other case, the arsenic concentration in rice was found to be high in Bangladesh [50].

Phosphate fertilization is suggested to lower the arsenate uptake in plants because both compounds enter the rice via the same transporter. However, there are arguments in certain cases because under flooding conditions, As is present as arsenite, which cannot compete with phosphate; furthermore, phosphate increases As mobility because it competes with arsenate for the adsorption site on Fe-oxides/hydroxides [51]. Presence of over 1.0&#;μg/g arsenic concentration in hair, 20 to 130&#;μg/g in nails, and over 100&#;μg per day in urine is an indication of arsenic poisoning [2]. Significant correlation was also observed with levels in human urine, toenail, and hair samples [31]. A meta-analysis assessing the effects of exposure to arsenic suggests that 50% increment of arsenic levels in urine would be associated with 0.4 decrement in the intelligence quotient (IQ) of children aged 5&#;15 years [52]. Arsenic uptake is adventitious because arsenate and arsenite are chemically similar to the required nutrients [53]. At neutral pH, the trivalent forms of these metalloids are structurally similar to glycerol, and hence they can enter cells through aquaporins [54].

4. Technologies/Methods for the Removal of Arsenic from Environment

According to World Health Organisation (WHO) standard set in the year , the maximum limit of arsenic contamination in drinking water is 10&#;μg/L or 10 ppb. [1]. This limit was later adopted by European Union in the year (council directive 98/83/EC), transposed by Portuguese legislation by Law Decree (DL) number 236/ [1, 55]. In the year , United States has also adopted the WHO standard for lowering the federal drinking water standard for maximum limit of arsenic from 50&#;μg/L to 10&#;μg/L [8]. Technologies for removing arsenic from the environment should meet several basic technical criteria that include robustness, no other side effect on the environment, and the ability to sustain water supply systems for long terms and meet the quality requirement of physical chemical, and microbiological approaches [1]. Currently, there are many methods for removing arsenic from the soil contaminated with arsenic, which could be divided into three categories, including physical, chemical, and biological approaches [14].

In the physical approaches, the concentration of arsenic in soil could be reduced by mixture of both contaminated and uncontaminated soils together that will lead to an acceptable level of arsenic dilution [14]. Soil washing is another method which is grouped under physical approaches whereby arsenic contaminated soil will be washed with different concentration of chemicals such as sulfuric acid, nitric acid, phosphoric acid, and hydrogen bromide [14]. The choice of chemicals used for extractant and high cost have often restricted the usage of soil washing into a smaller-scale operations as it is the disadvantages of using soil washing method [14]. Meanwhile, cement can immobilise soluble arsenites and has been successfully used to stabilise As-rich sludges which may be suitable for treating sludges generated from precipitative removal units [15]. Furthermore, the disposal of water treatment wastes containing As, with a particular emphasis on stabilisation/solidification (S/S) technologies, has been assessed for their appropriateness in treating As containing wastes. In this process, brine resulting from the regeneration of activated alumina filters is likely to accelerate cement hydration. Furthermore, additives (surfactants, cosolvents, etc.) have also been used to enhance the efficiencies of soil flushing using aqueous solutions as water solubility is the controlling mechanism of contaminant dissolution. The usage of surfactant alone gives about 80&#;85% of efficiencies in laboratory experiments. Studies indicated that when soil flushing is applied in the field, efficiency can vary from 0% to almost 100%. It often gives moderate efficiencies by using only one product (surfactant, cosolvent, and cyclodextrin). On the other hand, the use of more complex methods with polymer injection leads to higher efficiencies [16].

The current available chemical remediation approaches mainly involving methods such as adsorption by using specific media, immobilization, modified coagulation along with filtration, precipitations, immobilizations, and complexation reactions [1, 14]. The coagulation along with filtration method for removing arsenic from contaminated sources is quite economic but often displayed lower efficiencies (<90%) [1]. The formation of stable phases, for example, insoluble FeAsO4 (and hydrous species of this compound such as scorodite, FeAsO4·2H2O), is beneficial for the stabilization procedure [17]. Furthermore, the use of selective stabilizing amendments is a challenging task as the majority of polluted sites are contaminated with multiple metal(loid)s. Nanosized oxides and Fe(0) (particle size of 1 to 100&#;nm) are another possible enhancement for the stabilization method [17]. Natural nanoparticulate oxides are important scavengers of contaminants in soils [56] and due to their reactive and relatively large specific surface area (tens to hundreds m2/g), engineered oxide nanoparticles are promising materials for the remediation of soils contaminated with inorganic pollutants [18, 19]. It is reported that chemical remediation gained popularity because of its high success rate, but it could be expensive when someone would like to remediate a large area [14]. In contrast, biological remediation or bioremediation of soils contaminated with either inorganic or organic arsenic present in pesticides and hydrocarbons have been widely accepted in some places [14]. Even though bioremediation suffers several limitations, these approaches have been gaining interest for the remediation of metal(loid) contaminated soils due to their cost effectiveness [14]. Basically, bioremediation technology could be divided into subcategories: intrinsic bioremediation and engineered bioremediation [14]. Intrinsic bioremediation is generally referred to as the degradation of arsenic by naturally occurring microorganisms without intervention by human, and this method is more suitable for remediation of soil with a low level of contaminants [14]. Engineered bioremediation often relies on intervention of human for optimizing the environment conditions to promote the proliferation and activity of microorganisms that lived in that area. As a result, the usage of engineered bioremediation method is more favorable in the highly contaminated area [14].

Mechanism for arsenic detoxification can be divided into four which known as uptake of As(V) in the form of arsenate by phosphate transporters, uptake of As(III) in the form of arsenite by aquaglyceroporins, reduction of As(V) to As(III) by arsenate reductases, and extrusion or sequestration of As(III) [57]. AQPs have been shown to facilitate diffusion of arsenic [53, 54]. The microbial oxidation of As in Altiplano basins (rivers in northern Chile) was demonstrated by Leiva et al. [20]. The oxidation of As (As(III) to As(V)) is a critical transformation [58] because it favors the immobilization of As in the solid phase. As(III) was actively oxidized by a microbial consortium, leading to a significant decrease in the dissolved As concentrations and a corresponding increase in the sediment's As concentration downstream of the hydrothermal source. In situ oxidation experiments demonstrated that the As oxidation required a biological activity, and microbiological molecular analysis had confirmed the presence of As(III)-oxidizing groups (aro A-like genes) in the system. In addition, the pH measurements and solid phase analysis strongly suggest that As removal mechanism must involve adsorption or coprecipitation with Fe-oxyhydroxides. Taken together, these results indicated that the microorganism-mediated As oxidation contributed to the attenuation of As concentrations and the stabilization of As in the solid phase, therefore controlling the amount of As transported downstream [20]. Since most of the cases of arsenic poisoning are due to the consumption of water contaminated by arsenic, the process of cleaning up or reducing arsenic concentration in water becomes very important. Methods used in reducing arsenic levels in water are primarily divided into (i) physiochemical methods, which include filtration or coagulation sedimentation, osmosis or electrodialysis, adsorptions, and chemical precipitations and (ii) biological methods such as phytoremediation by using aquatic plants or microbial detoxification of arsenic [14].

Generally, two approaches are mainly employed in the phytoremediation method. The first approach uses &#;free-floating plants such as water hyacinth&#; that could adsorb metal(loid)s and the plants would be removed from the pond once the equilibrium state is achieved [14]. The second approach uses aquatic rooted plants (i.e., Agrostis sp., Pteris vittata, Pteris cretica, and others) to remove arsenic from bed filters and from water [14, 21&#;23]. Yang et al. [23] stated that the addition of arsenate reducing bacteria will promote the growth of P. vittata in soil. Two important processes in the removal of arsenic from water by microorganisms are biosorption and biomethylation [14]. It is reported that biomethylation (by As(III) S-adenosylmethionine methyltransferase) is the reliable biological process for removing arsenic from aquatic media [14].

Recently, the arsenite (As(III)) S-adenosylmethionine methyltransferase (ArsM) gene has been inserted into the chromosome of Pseudomonas putida KT for potential bioremediation of environmental arsenic [59]. The first structure of As(III) S-adenosylmethionine methyltransferase by X-ray crystallography was described by [60]. In this enzyme, there are three conserved cysteine residues at positions 72, 174, and 224 in the CmArsM orthologue from the thermophilic eukaryotic alga Cyanidioschyzon sp. [61]. Substitution of any of the three led to the loss of As(III) methylation [61]. The relationship between the arsenic and S-adenosylmethionine binding sites to a final resolution of ~1.6&#;Å. As(III) binding causes little change in conformation, but binding of SAM reorients helix α4 and a loop (residues 49&#;80) towards the As(III) binding domain, positioning the methyl group to be transfer to the metalloid [60].

5. Arsenic Resistant Microorganisms

Studies of bacterial growth at high arsenic-phosphorus ratios demonstrated that high arsenic concentrations can be tolerated relatively and that it can be involved in vital functions in the cell [62]. Corynebacterium glutamicum survives arsenic stress with two different classes of arsenate reductases. Cg-ArsC1 and Cg-ArsC2 are the single-cysteine monomeric enzymes coupled to the mycothiol/mycoredoxin redox pathway using a mycothiol transferase mechanism, while Cg-ArsC1' is a three-cysteine containing homodimer that uses a reduction mechanism linked to the thioredoxin pathway [63]. The presence of naturally occurring arsenate and arsenite in water and soil environment which could enter the cells by the phosphate-transport system has given pressure for microorganisms to maintain their arsenic detoxification systems for surviving purposes. One of the commonest forms of arsenic resistance in microorganisms is by detoxification operons, which are encoded on genomes or plasmids [64].

Most of the detoxification operons consist of three genes, which are known as arsC (reduction of arsenate to arsenite), arsR (transcriptional repressor), and arsB (can also be a subunit of the ArsAB As(III)-translocating ATPase, an ATP-driven efflux pump) [3, 53, 64]. Moreover, some detoxification operons also contain two additional genes (arsD-metallochaperone and arsA-ATPase) [3, 64]. The ArsD metallochaperone binds cytosolic As(III) and transfers it to the ArsA subunit of the efflux pump [53]. In normal process, arsenate that enters the cell will be reduced to arsenite by ArsC gene before it is transported out of the cell by ArsB gene [10, 64]. As a result, a more toxic form of arsenic will be introduced into the environment. In another study, Villegas-Torres et al. [65] indicated that the arsenic resistance ability in Bacillus sphaericus could be due to the presence of arsC gene, which could be horizontally transferred between microorganisms isolated from Columbian oil polluted soil that contain high arsenic levels.

The other reduction of arsenate to arsenite by microorganisms is via dissimilatory reduction mechanism that could be carried out in facultative anaerobe or strict anaerobe condition with the arsenate acting as the terminal electron acceptor [24]. These microorganisms have the ability to oxidize inorganic (sulfide and hydrogen) and organic (e.g., formate, aromatics, and lactase acetate) as an electron donor which will lead to the production of arsenite, and they were normally named dissimilatory arsenate-respiring prokaryotes (DARPs) [4].

Two other families of arsenate reductase are known as thioredoxin (Trx) clade and Arr2p arsenate reductase. It is reported that Trx clade is linked with arsC arsenate reductase gene while Arr2p is related to different class of larger protein tyrosine phosphatases [66]. Zargar et al. [67] reported that ArxA (arsenite oxidase) enzymes, which are present in Alkalilimnicola ehrlichii MLHE-1 strain (a chemoliautotroph bacteria) could couple with arsenite oxidation as well as nitrate reduction. Different types of bacteria with the ability of resisting arsenic are Rhodococcus, Arthrobacter, Acinetobacter, Agrobacterium, Staphylococcus, Escherichia coli, Thiobacillus, Achromobacter, Alcaligene, Pseudomonas, Microbacterium oxydans, Ochrobactrum anthropi, Cupriavidus, Desulfomicrobium, Cyanobacteria, Sulfurospirillum, Wolinella, Citrobacter, Agrobacterium, an arsenic reducing bacteria from Flavobacterium-Cytophaga group, Scopulariopsis koningii, Fomitopsis pinicola, Penicillium gladioli, Fusarium oxysporum meloni, Fucus gardneri, Bosea sp., Psychrobacter sp., Polyphysa peniculus, Methanobacterium, Bradyrhizobium, Rhodobium, Sinorhizobium, and Clostridium [13, 21, 53, 68&#;71].

Liao et al. [69] reported that 11 arsenic reducing bacteria strains from seven different genera (i.e., Pseudomonas, Psychrobacter, Citrobacter, Bacillus, Bosea, Vibrio, and Enterobacter) were isolated from environmental groundwater samples collected from well AG1 in Southern Yunlin County, west-central Taiwan. In Liao et al. [69] report, they indicated that diverse community of microorganisms holds a significant impact in the biotransformation of arsenic that is present in the aquifer, and these communities of bacteria are well adapted to high arsenic concentrations that are present in the water. In another study, Mumford et al. [72] reported that Alkaliphilus oremlandii and ferum reducing bacteria such as Geobacter species were present in arsenic rich groundwater beneath a site-specific site (C6) on Crosswicks Creek, New Jersey. Other bacteria with the ability of reducing arsenate to arsenite are Sulfurospirillum barnesii and Sulfurospirillum arsenophilum from the ε-proteobacteria as well as Pyrobaculum arsenaticum from Thermoproteales order and Chrysiogenes arsenatis [4, 10, 73, 74]. Afkar [10] reported that the reduction of arsenate to arsenite by S. barnesii strain SeS-3 is associated with the membrane cell where this resistance mechanism is encoded by a single operon that consists of arsenite ion-inducible repressor. Besides that, Afkar [10] also indicated that S. barnesii strain SeS-3 reduced arsenate to arsenite under anaerobic condition using arsenate as terminal electron acceptors while lactate as the carbon source.

In another study, Youssef et al. [75] reported that both Neisseria mucosa and Rahnella aquatilis are able to reduce arsenate and selenate. In their study, both N. mucosa and R. aquatilis were grown in a neutral pH medium (pH 7) containing five mM sodium arsenates where the sodium lactate acts as an electron donor while N. mucosa and R. aquatilis act as the electron acceptor organisms. Although both N. mucosa and R. aquatilis strains studied are able to grow at higher pH medium (pH10), their growth rate decreased drastically (reduction of 43% in N. mucosa and 67.2% in R. aquatilis) and has been observed [75]. Meanwhile, archaebacterium Sulfolobus acidocaldarius strain BC, Alcaligenes faecalis, Shewanella algae, β-proteobacteria strain UPLAs1, Alcaligenes faecalis, Comamonas terrae sp. nov, some heterotrophic bacteria (Herminiimonas arsenicoxydans), and chemoliautotrophic bacteria are reported to have the ability to oxidize arsenite to a less toxic arsenate [4, 14, 25&#;28]. In this case, arsenite will often serve as an electron donor for reducing the nitrate or oxygen that will produce energy in order to fix carbon dioxide [26]. Two genes, aoxA and aoxB encoding for arsenite oxidase played an essential role in the oxidation of arsenite into arsenate [27]. The insertion of mini-Tn5::lacZ2 transposon in aoxA or aoxB gene will stop to arsenite oxidation process [27].

In another review by Silver and Phung [12], they indicated that both asoA and asoB genes which encoded for large molybdopterin-containing and small Rieske (2Fe-2S) cluster the subunit of oxidation of arsenite in Alcaligenes faecalis. They identified that the upstreams of asoB consist of 15 genes while the downstreams of asoA consist of six genes, which are involved in arsenic resistance and metabolisms [12]. Besides bacteria, certain species of algae such as Fucus gardneri and Chlorella vulgaris are also known to have the ability to accumulate arsenic [76, 77].

Table 1 shows the advantages and disadvantages of physical, chemical, and biological methods for the removal of arsenic compounds. Physical method exhibits the simplest choice, but it was however limited to small scale operations. Chemical method had gained popularity by its high success rate; however, the remediation area can be exposed to other types of chemical contaminants. The usage of biological and phytoremediation methods might be the most practical methods for a small area but more research needs to be carried out especially in methylations, reduction, and oxidation using microorganisms for more effective method to remove the arsenic compound as they have a high potential application in the future.

Table 1.

Advantages and disadvantages of methods for the removal of arsenic compounds.

Method Method in Detail Advantages/Disadvantages Reference Physical approaches Mixing both contaminated and uncontaminated soils High cost/usage to smaller-scale operations [14] Physical approaches Washed with sulfuric acid, nitric acid, phosphoric acid, and hydrogen bromide Chemicals usage/high cost/usage to smaller-scale operations [14] Physical approaches Immobilise soluble arsenites using cement Successfully used to stabilise As-rich sludges [15] Physical approaches Emphasis on stabilisation/solidification (S/S) Treating As containing wastes in water [15] Physical approaches Soil flushing using aqueous solutions using surfactants and cosolvents Applied in the field, efficiency can vary from 0% to almost 100% [16] Chemical remediation approaches Adsorption by using specific media, immobilization, modified coagulation along with filtration, precipitations, immobilizations, and complexation reactions Economic but often displayed lower efficiencies (<90%) [1, 14] Chemical remediation approaches Formation of stable phases, for example, insoluble FeAsO4 (and hydrous species of this compound such as scorodite, FeAsO4.2H2O) Use of selective stabilizing amendments is a challenging task [17] Chemical remediation approaches Stabilization method using nanosized oxides and Fe(0) (particle size of 1 to 100&#;nm) Gained popularity/high success rate, but it could be expensive when remediating a large area [14, 17&#;19] Intrinsic bioremediation Degradation of arsenic by naturally occurring microorganisms More suitable for remediation of soil with a low level of contaminants [14] Engineered bioremediation Optimizing the environment conditions to promote the proliferation and activity of microorganisms Favorable method used in high contaminated area [14] Microbial oxidation Immobilization of As in the solid phase Required biological activity, and microbiological molecular analysis/involved adsorption or coprecipitation with Fe-oxyhydroxides. [20] Physiochemical methods Filtration or coagulation sedimentation, osmosis or electrodialysis, adsorptions, and chemical precipitations Widely accepted in some places [14] Biological methods Such as phytoremediation by using aquatic plants or microbial detoxification of arsenic Widely accepted in some places [14] Phytoremediation method Using &#;free-floating plants such as water hyacinth&#; Widely accepted in some places [14, 21&#;23] Using aquatic rooted plants such as Agrostis sp., Pteris vittata,&#;&#;and Pteris cretica Methylations Biomethylations (by As(III) S-adenosylmethionine methyltransferase) Is a reliable biological process of removing arsenic from aquatic mediums [14] Reduction Reduction of arsenate into arsenite by microorganisms via dissimilatory reduction mechanism Should be carried out in facultative anaerobe or strict anaerobe condition [24] Oxidation Using heterotrophic bacteria and chemoautotrophic bacteria to oxidize arsenite into a less toxic arsenate Should be carried out in controlled environment [4, 14, 25&#;28] Open in a new tab

Previously, bacterial biosensors (whole-cell) were being used to detect inorganic arsenic [78]. Biosensor technology was widely studied by using potentiometry, amperometry, and conductometry [79&#;81]. Only a few studies were carried out based on capacitometry [82] especially by using DNA and antibodies [83, 84]. Previously, capacitive sensor using enzyme was introduced for toxin detection [82]. A biosensor selective for the trivalent organoarsenicals methyl arsenate and phenyl arsenite over inorganic arsenite was reported by Chen et al. [78] which may be useful for detecting degradation of arsenic-containing herbicides and growth promoters. A surface plasmon resonance biosensor for the study of trivalent arsenic was also reported by Liu et al. [85]. This biosensor indicates that the 3D hydrogel-nanoparticle coated sensors exhibited a higher sensitivity than that of the 2D AuNPs decorated sensors. It was shown that binding of As(III) into ArsA was greatly facilitated by the presence of magnesium ion and ATP. For future research, biosensor based on capacitometry using enzymes [82] such as As(III) S-adenosylmethionine methyltransferase for arsenic detection remains interesting to be explored.

6. Conclusion

Arsenic is a metalloid that causes harm to humans and environments. However, certain species of prokaryotes have the abilities to use arsenic either through oxidation or reduction process for energy conservation and growth purposes. It is important to remove and reduce this pollutant from the environment through different approaches such as physical, chemical, and biological. The use of bioremediation to remove and mobilize arsenic from contaminated soils and aquifers could be an effective and economic way since a wide range of microorganisms have been found to be successfully degrading this pollutant from the environment.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

To address its potential, this review provides an overview of arsenic&#;s various forms and distribution in Cu ore and systematically explores arsenic treatment processes, predominantly focusing on typical treatment processes for ACC minerals. Following this, we outline the development trajectory of As extraction technologies that may be applied to minerals containing this compound, offering insights into the future possibilities for the efficient and safe utilization of ACC ores.

Nonetheless, as copper ore reserves deplete and mineral grades decline in copper smelting, the cost per unit of mined mineral escalates [ 11 12 ]. Rising operational costs prompt a reevaluation of the value of ACC minerals, despite their health and environmental hazards. From this standpoint, effectively and safely removing arsenic from copper minerals now seems to hold significant importance in fostering the sustainable and healthy development of China&#;s mineral raw material industry [ 12 ].

However, the processes for smelting ACC minerals generate various harmful substances, among which arsenic stands out as a common and particularly hazardous compound [ 13 ]. Arsenic&#;s toxicity to the human body can cause acute poisoning upon short-term exposure and can potentially lead to cancer upon prolonged contact [ 14 ]. Furthermore, the presence of arsenic poses threats not only to human health but also to the environment [ 13 ]. Arsenic exists widely in various minerals such as arsenopyrite, chalcopyrite, and tetrahedrite, and its presence in mineral formations such as these substantially diminishes the value of their Cu ores.

With societal progress and economic advancement, copper&#;s applications have expanded across diverse human endeavors. Copper has been used in integrated circuit (IC) devices in sizes less than 180 nm due to its low resistivity [ 1 ], resulting in a significant global surge in copper consumption. To meet the escalating demands, copper smelting capacity has also experienced rapid expansion, with a consistent annual increase in smelting scales. Nonetheless, the finite nature of copper resources poses a challenge, intensifying the reliance on Cu concentrates. Accordingly, a historic surge in the capacity of refined Cu has been witnessed in recent years [ 2 6 ]. Concurrently, the continued increases of resource utilization have resulted in a notable decline in high-quality Cu reserves. Global reports indicate a decrease in the average quality of Cu ores from 1.6% in to approximately 1.0% in recent years [ 7 9 ]. In China, more than half (56%) of Cu resources exhibit qualities under 0.7%. Against this background, as Cu production continues and high-grade Cu mineral resources diminish, Cu concentrates with low As contents decrease each year and there is a growing focus on As-containing Cu (ACC) minerals [ 10 12 ], which will inevitably be utilized for Cu smelting.

Given the increasing efforts being made to ensure environmental protection and develop sustainable strategies, the treatment of As-waste in Cu metallurgy is becoming a high priority, although there are costs associated with research and development [ 29 30 ]. Currently, arsenic smelting manufacturers have not yet devised an effective treatment method. While the options are being explored, large amounts of As waste are being discharged in three forms: gas, liquid, and solid. As shown in Table 1 , according to United States Geological Survey (USGS) data, the annual output of arsenic is huge and growing [ 31 32 ]. However, when analyzing the data of the world&#;s two largest economies, the United States of America (USA) and China, it is found that the import and export quantities are relatively stable and account for only one-fifth of the output, which reflects the accumulation of this toxic waste and underscores the pressing short-term need for a solution. Until one is found, non-ferrous metallurgical enterprises will continue to have a highly adverse impact on their surrounding environments [ 13 14 ].

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In Cu metallurgy, along with the obvious shortcoming of Cu resources at present, an accompanying issue that must be faced is the increasing use of complex materials, particularly ACC minerals. There are options for processing these complex materials, such as residual arsenic treatment, but the costs of such approaches are increasing sharply. Furthermore, regardless of the Cu mineral properties and the sorting techniques used, it remains especially difficult to entirely separate As from Cu minerals, resulting in concentrates that contain amounts of As. As is severely toxic to human beings and other creatures. It causes direct damage to the nerve cells of the human body and can lead to peripheral neuritis, multi-organ carcinogenesis, and damage to the heart, liver, and other parenchymal organs [ 23 ]. The lethal dose for a human via acute poisoning is 0.2&#;0.6 g of arsenic [ 24 ]. Beyond its human impact, arsenic also pollutes the environment, necessitating investments in arsenic treatment [ 25 ]. Figure 2 presents a diagram of the chemical cycle of arsenic worldwide [ 26 28 ].

This scenario presents a challenge in meeting the demand for copper production, as demand continues to grow while resources decline. One potential solution is the utilization of ACC resources. An ACC concentrate is the term used for a Cu concentrate containing more than 2% As. The resources to produce such a concentrate are readily available, with the proportion of As to Cu in 15.0% of Cu resources standing at 1:5. Among these, tetrahedrite (CuAs) and arsenate (CuAsS) are common As-bearing Cu deposits. While such Cu resources were previously overlooked, they are increasingly being considered as main materials for Cu metallurgy [ 21 22 ].

Recently, the intensified utilization of copper resources has significantly depleted high-quality and easily sorted Cu reserves [ 15 ]. Accordingly, reports indicate a pronounced decline in the average global Cu mineral grade, reducing from 1.6% in to around 1.0% in current assessments. This decrease is accompanied by a diminishing availability of high-grade Cu. Figure 1 visually tracks world Cu ore grade changes from to [ 16 17 ], with the graphical representation offering a clear illustration of the declining trend, emphasizing the tangible and concerning effects of prolonged and escalating resource exploitation on ore quality [ 18 20 ].

Subsequently, the introduction of wet smelting techniques revolutionized the process by utilizing chemical reactions to convert arsenic into soluble materials for extraction, thus markedly enhancing the removal efficiency. Wet arsenic removal methods boast advantages such as minimal air pollution and a high resource recovery rate, significantly aiding in Cu concentrate recovery. Nonetheless, a major drawback lies in the treatment of As-containing wastewater, which often necessitates additional As solidification, with associated costs typically surpassing those involved in treating As-containing waste gas [ 38 39 ]. Beyond these, recent advancements in biometallurgy have presented innovative solutions employing specialized microorganisms for the bioleaching or biosorption of As. However, large-scale industrial application remains theoretical, lacking specific practical implementation. Overall, this chronological progression from physical to chemical and biological methods reflects a continuous quest for more effective, environmentally sustainable, and thorough arsenic removal techniques in the realm of ACC mining.

The evolution of As removal technology in ACC mining has undergone several phases, marked by advancements in techniques aimed at efficiently eliminating As from ore. Initially, early methods relied on mechanical separation processes such as sorting and flotation, supplemented by pyrometallurgy. Pyrometallurgical processes offer simplicity, primarily involving roasting, and they have widespread applicability across various ACC minerals. However, a notable disadvantage is the challenging collection of exhaust gas, leading to high treatment costs due to environmental regulations governing waste gas management [ 33 37 ].

Based on the above-mentioned process, Putra et al. proposed using sodium carbonate and arsenic fixed roasting to convert arsenic (CuAsS) to sodium As, in which arsenic is dislodged at a temperature of 800 °C. This approach is described through the following Equations (8) and (9) [ 52 ].

Previous reports have stated that when the roasting temperature ranges from 400 to 550 °C, chalcopyrite (CuAsS) undergoes a sulfation process, forming Cu sulfate, Cu, As, ferrum (Fe), sulfate (S), Fe, and FeAsO. In this process, over 80% of As is fixed in the state of arsenate, and it only evaporates when the temperature rises above 650 °C [ 51 ].

Adham K and Harris CT analyzed the relationship between the equilibrium amount of gaseous As and the Owithin the As-O-S system at 700 °C; their results are given in Figure 3 49 ]. This reveals whether arsenic&#;s presence may be determined based on the oxygen potential and shows that arsenic mainly exists in three gaseous forms: As, As, and As. Meanwhile, when the amount of oxygen increases, a considerable quantity of sulfur evaporates in the form of SO(shown in Figure 3 ) [ 50 ].

In general, a high oxygen level results in the formation of Cu 3 As and CuO phases, simultaneously transforms As 2 O 3 into As 2 O 5 , and, finally, produces a copper-arsenate compound (3CuO·As 2 O 5 ) with CuO in the condensate. This process is not conducive to arsenic removal.

Wilkomirsky et al. [ 45 ] considered the possibility of neutral roast reactions for high-As Cu minerals and developed an Ellingham diagram accordingly. This was part of an attempt to simulate the thermodynamics and kinetics of the fluidized-bed neutral roasting of an enargite concentrate at the University of Concepcion&#;s pilot plant (Rajoria et al. [ 46 ]). In this simulation, the vast majority of the roasting reactions are exothermic. Enargite is directly broken down into CuS (s) and As(g), which can also be achieved through a reaction with FeS.

Moreover, a method that uses carbon monoxide to supply a reducing atmosphere may offer a useful means of As removal. In this pathway, when the COcontent reaches 15.5% in the exhaust gas, the As amount is decreased from 9% to 0.2% [ 44 ].

During anaerobic situations, chalcopyrite (CuAsS) is initially decomposed at 550 °C, as described in Formula (1), reported previously [ 40 43 ]. Meanwhile, CuS is unstable at temperatures from 500 °C to 700 °C and changes into CuS, as described by Formula (2) [ 40 43 ].

The application of pyrometallurgy for As removal involves separating arsenic in its gaseous form from ACC mines. This may be achieved through the use of different roasting atmospheres, which includes reduction, oxidation, and sulfate/carbonate pyrometallurgies [ 15 ].

In summary, the acid extraction As removal method is actually a means of Cu extraction with a high rate of As removal, and As is solidified when carrying out this leaching method. However, a certain amount of waste is generated, and this method requires the addition of oxidants, which results in high costs in industrial production.

A previous report presented a study on high-As Cu minerals in Fe(SO4)media. The study showed that the arsenic leaching rate of high-As Cu minerals in Fe(SO4)medium, with a temperature of 80 °C and excessive Fe 64 ], reached 100%. Furthermore, in a study on copper smelting, the As removal rate was close to 96% when polytetrafluoroethylene (PTFE) was added [ 65 66 ], in a process that also accelerated the change of arsenic into smectite, thus obtaining an arsenic solidification compound.

The leaching mode of high-As Cu minerals involves using reagents including HSOand HCl to removal As (III) compounds [ 60 61 ]. In this method, a series of oxidants including H, O, and Femust be added to the reaction setup to promote the reaction of As (III) with the oxidant. When taking this approach, the Cu in high-As Cu minerals changes into Cuand Cu(AsOthrough the adjustment of the pH. This is represented by chemical Equations (13) and (14) [ 62 ]. Additionally, to promote the As leaching process, measures are taken such as adding NaCl or Fe, regulating the pH, and raising the temperature [ 63 ].

In a previous report, the results of a serial leaching experiment performed on high-As Cu minerals using the NaClO-NaOH system were presented. In this study, the experiment comprised a ratio of liquid/solid of 5:1, a temperature of 50 °C, and HClO concentration of 10%; the As removal rate was over 80% [ 58 ]. In another study, NaClO (0.13 mol/L) and NaOH were used to jointly act on chalcopyrite: a one-hour leaching experiment was conducted at 40~60 °C, and the As removal rate was over 80% [ 59 ]. In sum, applying the proposed alkaline leaching method to removal arsenic requires an alkaline leaching solution and strongly alkali-resistant equipment, and the As removal rate is only 80.0%.

A leaching study was conducted on chalcopyrite when using this system, which showed that chalcopyrite may be decomposed into CuS, and arsenic may be removed at a rate exceeding 90% at a temperature of 80 °C and a pH of 12.5 [ 57 ].

This system has been applied in the treatment of arsenic tetrahedrite [ 55 ]. Under the conditions of a NaS concentration of 90&#;100 g/L, NaOH concentration of 70&#;80 g/L, a temperature 105 °C, and a liquid/solid ratio of 5:1, the rate of As removal can surpass 91.0%. Furthermore, using the approach mentioned above, Ruiz MC and Grandon L probed the co-effect of NaS and NaOH on chalcopyrite leaching. In doing so, they found that a rate of arsenic removal of over 97% could be achieved through the excessive use of NaS and NaOH at a temperature of 80 °C [ 56 ].

Alkali leaching is a frequently used method of As removal, in which the Cu in chalcopyrite is converted into CuS or CuO by regulating the pH and increasing oxidants. Meanwhile, As is also changed into AsSor/and AsO 54 ]. The current alkaline leaching systems mainly include NaS-NaOH, NaHS-NaOH, and NaClO-NaOH.

One study conducted a leaching experiment on high-As Cu minerals using mesophilic acidophilic, thermophilic acidophilic Thiobacillus, and thermophilic. The study indicated that such bacteria have an accelerating effect on the extraction of arsenic [ 68 ]. In addition, a leaching experiment on ACC mines was conducted using moderately thermophilic bacteria. This experiment showed that under the conditions of pH 1.5 and 55 °C, when the slurry concentration was set as 5.0%, the As removal rates reached 78.21% [ 69 ]. Collectively, the research indicates that the biometallurgy method is a low-cost and low-polluting Cu extraction approach, but the As removal rate is relatively low.

Generally, biometallurgy for As removal include two stages [ 67 ]. In this method, the first stage is dependent on the direct action of bacteria, which leads to the extraction and dissolution of As and S in HAsOand HSO, respectively [ 15 ]. This reaction is described in Formula (15). In the second stage, the indirect reaction of bacteria dominates the entire process and accelerates the reaction set out in Equation (16).

Notably, Outotec in Frankfurt, Germany, manages a pilot plant employing roasting techniques to treat Cu concentrates, operating at a capacity of 25 kg/h. Beyond this, a few smelters process enargite concentrates directly, though most limit their arsenic intake for environmental reasons, preferring cleaner Cu concentrates (<0.5% As) [ 57 ]. The ASARCO (American Smelting and Refining Company) smelter in El Paso, Texas accepts Cu concentrates with a maximum As content of 0.2% [ 61 ]. Meanwhile, the Lepanto roaster of the Lepanto Consolidated Mining Company, the Philippines, treats about 180 t/d of enargite concentrates containing 11% As, 31% Cu, 15% Fe, and 34% S, and produces calcine containing 0.3% As, 43% Cu, 23% Fe, and 20% S [ 70 ]. Elsewhere, the Boliden roaster in Boliden Oy, Finland, with a capacity of up to 800 t/d, treats concentrates containing 1~2% As and typically produces a calcine containing about 0.1% As [ 57 ]. Dundee Precious Metals in Bulgaria, meanwhile, transports 175,000 t/a enargite concentrates to the NCS (Namibia Custom Smelters) smelter in Tsumeb, Namibia, known for processing high As and lead-bearing Cu concentrates, producing copper blister and arsenic trioxide (As2O3) [ 62 ].

The processing of the Ministro Hales deposit in Chile by Codelco [ 59 60 ] commenced in with an annual throughput of 550,000 tons of Cu concentrate and concurrent production of 250,000 tons of sulfuric acid. This operation implements controlled oxidation, also termed neutral roasting, which is aimed at extracting arsenic in vapor form. The As-containing dust generated in the process undergoes collection and subsequent conversion into a disposable form or is stockpiled for management.

Oudenne [ 58 ] reviewed CMI NESA&#;s (Chartered Management Institute, NSW Education Standards Authority) three-decade-long efforts to treat arsenic, antimony-bearing concentrates, and other complex metallurgical byproducts. The review emphasized preliminary mineralogical characterization as being crucial for successful roasting. Their selective roasting, especially under reducing atmospheres, proved effective for removing arsenic from Cu-bearing materials such as tennantite, enargite, chalcopyrite, and pyrite. Variable factors, such as particle size, significantly influence the volatilization of cadmium (Cd), As, stibium (Sb), bismuth (Bi), and hydrargyrum (Hg) during this roasting. This selective roasting process aims for impurity removal, requiring specific conditions such as optimal temperature, a controlled heating rate, and a reducing atmosphere. However, employing a roasting furnace with a single atmosphere posed challenges for complex sulfide materials. Nonetheless, it was possible to eliminate volatile sulfides from nonvolatile minerals by using a reducing atmosphere. In a multi-stage reactor, independent temperature and atmosphere control in each hearth proved crucial for effective processing both above and within the material bed.

In an example of the industrial-scale removal of arsenic, the EI Indio (Chile Indio) [ 57 ] operation in Chile processed enargite concentrates comprising 23 wt.% Cu, 15 wt.% Fe, 35 wt.% S, 10 wt.% As, and 0.8 wt.% Sb. These concentrates underwent treatment in a 14-hearth, 6.5-m diameter Nicholas Herreschoff roaster, operating within the temperature range of 500 to 700 °C while maintaining an oxygen content below 0.5%. This operational setup ensured a state of essentially neutral roasting. The El Indio roaster&#;s process flow is illustrated in Figure 4 . The feed material was roasted within the roaster for approximately 3 h. The resultant outlet gases were collected using two cyclones, with the retrieved dust reintroduced into the roasting process and unoxidized gases (comprising sulfur and arsenic trisulfide) channeled into a combustion chamber. Here, excess air facilitated an exothermic reaction at temperatures of around 750 ± 25 °C. Following oxidation, the gases underwent cooling to 350 to 400 °C within a heat exchanger before entering an electrostatic precipitator. The dust, enriched with high-value precious metals, was blended with flotation concentrates. The dust-free gases then underwent additional cooling to roughly 120 ± 5 °C using cold air, prompting the condensation and removal of a significant portion of arsenic as arsenic trioxide, which took the form of a white powder within the gas phase. The resultant gas stream was ultimately discharged into the atmosphere.

The United States Environmental Protection Agency (USEPA) has designated the precipitation of dissolved arsenic using ferric ions as &#;The Best Demonstrated Available Technology&#; (BDAT) [ 64 ]. The stability of any disposed As compound is influenced by various factors including the disposal location&#;s characteristics, the crystallinity and size distribution of the compound, and the presence of oxygen or any complexing agents.

As removal from solutions containing both dissolved Fe and As is commonly achieved through co-precipitation using ferrihydrite precipitation techniques [ 63 ]. Post-precipitation involves the elimination of As from solutions by introducing an aqueous solution containing ferric-bearing species at a pH range conducive to ferrihydrite formation (typically pH 7 to 8). Another method, adsorption, entails the extraction of arsenic from solutions through exposure to previously precipitated ferrihydrite solids. The effectiveness of precipitation is contingent upon various factors such as duration, temperature, pH levels, the mole ratio of Fe/As, agitation rate, the arsenic&#;s valence state, and associated ions, among others.

9. Conclusions and Prospects

This study presented a comprehensive overview of the global copper landscape, shedding light on the escalating demand for copper and the formidable challenges stemming from diminishing high-grade copper resources. The rank of Cu ores has decreased from 1.6% to approximately 1.0%. The resulting rise in ACC minerals has surfaced as a significant concern. The proportion of As and Cu in 15.0% Cu resources is currently close to 1:5. The prevalence of arsenic in these minerals has raised concern regarding environmental and health hazards in Cu smelting operations.

Diverse methodologies, encompassing pyrometallurgy, hydrometallurgy, and biometallurgy, have been explored for As removal, with each method exhibiting distinct advantages and limitations. Pyrometallurgy, predominantly relying on roasting at high temperatures, encounters obstacles related to efficient gas-solid separation and environmental contamination. Hydrometallurgic techniques, meanwhile, such as alkaline and acid leaching, offer viable alternatives, albeit requiring expensive additives and posing challenges in solidifying arsenic in solution. Beyond these, biometallurgy, employing bacterial processes, emerges as a greener approach, but suffers from comparatively lower arsenic removal rates. Additionally, specific industrial As removal methods deployed in Cu smelting globally, such as neutral and selective roasting in various smelters, strive to curtail arsenic contents. However, substantial hurdles persist in achieving high-purity arsenic trioxide and implementing efficient gas-solid separation techniques.

As entering the smelting system can bring hazards to the smelting process, the process of treating arsenic after it enters the smelting system is extremely cumbersome. It is therefore essential to purify and remove As from the ore before it enters the smelting system. At this stage, there are two options for processing arsenic-containing ores: the wet method and the pyrometallurgical method. However, if the wet method is used to remove As, the As entering the solution cannot be effectively utilized, which is not useful for treating As pollution. The pyrometallurgical method mainly uses roasting to remove arsenic. The biggest challenge is associated with the high-temperature segregation of As-containing gas and dust, achieving gas-solid segregation in high-temperature conditions to obtain high-purity As trioxide and thereby realizing the resource utilization of arsenic. At this point, there are very few methods for gas-solid separation, such as electrostatic precipitation, baghouse filtration, and solid membrane filtration.

Electrostatic precipitation has limited collection ability for fine particles, and the smaller the particles, the higher the energy consumption. Baghouse filtration has the risk of bag burning at high temperatures. Solid membrane filtration includes various types of metal ceramics, generally resistant to high temperatures. Moreover, filter cake formed during the filtration process can effectively collect fine particles. Finally, the amount of energy consumed during nitrogen backflushing is much lower than that of electrostatic precipitation. Therefore, using solid membrane filtration can be said to be the only option for dust removal from roasting flue gas.

In summary, since the difficulty of handling arsenic is significantly increased after it enters a solution, we propose that using gas filtration is the most economical and effective method for separating arsenic from smoke. The electrostatic precipitation process used by EI Indio can effectively remove arsenic and obtain arsenic oxide at the same time. Therefore, when seeking to improve gas solid separation methods, the use of high-temperature porous membrane separation can not only improve the purity of arsenic oxide after separation but can also save electricity. Accordingly, we predict that this will be the direction taken for ACC processing in the future.

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