Although arsenic (As) occurs ubiquitously in the environment and has been used since its isolation in 1250 A.D in various fields such as medicine, metallurgy, agriculture and electronics, it is undoubtedly best known for its toxicity to both plants and animals. The toxic effects of arsenic in humans range from skin lesions to cancer of the brain, liver, kidney and stomach. Generally inorganic arsenic species are more toxic to humans and other animals than organic forms. The oral LD50 (dose lethal to 50% of subjects) for inorganic arsenic ranges from 15-293 mg -kg-1 and 11-150 mg -kg-1 bodyweight in rats and other laboratory animals respectively, and inorganic arsenicals are proven carcinogens in humans. However, the wide range of arsenic toxicity that has been determined depends on arsenic speciation.
Arsenic exists in four oxidation states, +V (arsenate), +III (arsenite), 0 (arsenic), and –III (arsine). More than 245 minerals contain arsenic and although the ultimate source of arsenic is geological, human activities such as mining, the burning of fossil fuels and pesticide application can also cause arsenic pollution. In addition to arsenite, arsenate, and their methylated derivatives, “fish arsenic” (arsenobetaine, AB and arsenocholine, AC) and arsenosugars of environmental interest are found in organisms, soils, sediments and natural waters. The molecular formulae of organic arsenic compounds found in the environment are given in Table 1.
Arsenite (iAsIII) is usually more toxic than arsenate (iAsV). Recent studies have found that monomethylarsenous acid and dimethylarsenous acid (MMAIII and DMAIII) are more acutely toxic than iAsV, MMAV and DMAV. The toxicity of trivalent arsenic is also related to its high affinity for the sulphhydryl groups of biomolecules such as glutathione (GSH) and lipoic acid and the cysteinyl residues of many enzymes. The formation of As(III)-sulphur bonds inhibits the activities of enzymes such as glutathione reductase, glutathione peroxidases, thioredoxin reductase and thioredoxin peroxidase. DMAIII is also known to form complexes with sulphur-rich proteins. However, sulphide-activated pentavalent arsenic can also bind to the sulphhydryl group of GSH and cause toxic effects.
Although arsenic is found ubiquitously in the environment, it is of especial concern in aquatic systems because of its potential to contaminate drinking water and food supplies. In seawater, the concentration of arsenic is usually less than 2 µg L-1 The levels of arsenic in unpolluted surface water and groundwater typically vary from 1-10 µg L-1. In freshwater, the variation is in the range of 0.15 – 0.45 µg L-1 although in thermal waters, concentrations of 8.5 mg L-1 and 1.8 – 6.4 mg L-1 have been reported in New Zealand and Japan, respectively. Natural geological sources of As into drinking water are one of the most significant causes of arsenic contamination around the world. Arsenic contamination in different parts of the world is summarised in Table 2. The World Health Organisation (WHO) has set a guideline of 10 µg L-1 as the maximum permitted level in drinking water. As Table 2 suggests, arsenic toxicity is a significant health risk for people in many different countries. As many as 60-100 million people globally may be at risk of exposure to excessive levels of arsenic.
The chemical speciation of arsenic in aqueous systems is of major importance when considering toxicity and/or measuring arsenic concentrations, and is affected by many environmental parameters. One key parameter is pH. Of the several forms of arsenic, the acidic forms of arsenite and arsenate as well as MMAV, and DMAV demonstrate acid-base equilibria, so different major and minor species are present depending on pH. As(OH)3 dissociates sequentially in water according to Equations 1-3.
At neutral pH, As(OH)3 is the dominant species while As(OH)2O- represents a small fraction (< 1.0%), and the contribution of As(OH)O2- and AsO3- is insignificant. As(V) also exists as a triprotic acid AsO(OH)3 and dissociates in a similar fashion with pKa values of 2.3, 6.8 and 11.6. At pH 7, almost equal concentrations of AsO2(OH)2- and AsO3(OH)2- are present. MMAv and DMAv are diprotic and monoprotic acids, respectively [Equations 7-9].
The major species of MMAV is CH3AsO2(OH)- at a neutral pH, but the minor species, CH3AsO32- is also present. In the case of DMAV, both (CH3)2AsO(OH) and (CH3)2AsO2- exist at pH 7.
The effects of pH, Eh (redox potential), adsorbing surfaces, biological mediation, organic matter and key inorganic substances such as sulphide, carbonate and phosphate combine in a complex and interwoven dynamic fashion to produce unique assemblages of arsenic species in water. Adsorption of dissolved arsenic onto particulate phases has been actively studied not only because of its pivotal role in determining As concentrations and speciation in natural waters, but as an important remediation tool for arsenic removal in contaminated drinking water. Iron oxides and hydroxides of a variety of composition and degree of crystallinities are virtually ubiquitous in natural aqueous systems and are known to play a major role in As geochemistry. The effect of organic matter (OM) on As adsorption, speciation and mobility is also a topic of keen interest because of the ubiquitous nature of natural organic matter in aqueous systems. Functional groups associated with OM can be involved in As speciation due to (i) possible redox reactions of As, (ii) organic matter coatings on inorganic adsorbents and (iii) aqueous complexes of As species. In many instances, high concentrations of organic matter in natural waters coincide with reducing and/or highly sulphidic conditions. These conditions are often found in ground water and thus their effects on As speciation may be critical with respect to the potability of water supplies.
Most of the organic compounds given in Table 1 have been found in marine and terrestrial systems. While the occurrence of organoarsenic compounds in fish and other aquatic fauna and flora is well known, arsenobetaine, which is the most commonly reported organoarsenical in marine animals, is virtually absent in vetebrate and invertebrate freshwater organisms. This represents the major difference in arsenic speciation between marine and freshwater organisms. Recently, arsenolipids have been found in cod fish oil resulting in concerns for human health.
Arsenosugars are found in marine algae and in marine animals such as scallops feeding on algae, and have also been identified in marine and freshwater fish and mussels. While arsenosugars are assumed to be relatively nontoxic to animals and humans compared with inorganic arsenic species, biotransformations of arsenosugars can result in toxic arsenicals. Biotransformation of arsenosugars in humans has been found to produce DMAV as a major metabolite in urine. Arsenosugars, though of considerably lesser toxicity than their inorganic counterparts, are of special interest because they are widespread in many different aquatic organisms, including many food sources. Arsenobetaine (AB) is very commonly found in seafood while MMAV and DMAV are the most commonly reported degradation products of AB formed upon cooking aquatic organisms.
Concerns over possible changes in arsenic levels of foods upon cooking have led to numerous studies which have indicated both increases and decreases in total arsenic levels in fish and bivalves such as mussels upon thermal preparation. Some of these changes may be due to loss of moisture upon heating as per microwave oven preparation or to volatilisation of arsenic species. Recently, concerns have also been raised about consumption of arsenic in cooked aquatically grown plant species such as seaweed and rice.
The ultimate solution to the contamination of food and water by arsenic due to anthropogenic activities, other than the cessation of the release of arsenic into the aquatic environment, is its removal by water treatment. The most common method of arsenic removal from water and wastewater is the use of adsorbents. The treatment of As-contaminated water by the adsorption of As with common coagulating agents (salts such as ferric chloride and ferric sulphate) has been studied extensively, revealing maximum As removal (over 95% of arsenate removed) with ferric sulphate. Other adsorbents include activated carbon and alumina, ion-exchange resins, sand, silica, clays, iron, iron compounds and organic polymers. Iron-based adsorbents (IBS) appear to be an emerging treatment method for removal of arsenic and have the advantage of strong affinities for arsenic at neutral pH.
Phytoremediation (plant based technology) for arsenic remediation is based on hyperaccumulation of arsenic by plants due to the interaction of As with high-affinity chelating molecules present in the cytoplasm of some plants. Arsenic accumulation in hyperaccumulating fern species may involve arsenic reductase and superoxide dismutase enzymes. Various biological processes such as plant-microbe interactions can also affect phytoremediation efficiencies. Recent molecular studies suggest that single genes and multigenic engineering approaches may be applied to enhance the efficiency of phytoremediation.
Under typical anoxic groundwater conditions, at near neutral pH, AsIII is the predominant form of arsenic, while in oxic groundwater, AsV dominates. Arsenite has a low affinity for mineral surfaces, while arsenate easily adsorbs to solid surfaces. An oxidation/precipitation technology should thus be very effective for the removal of arsenic from water. Many studies have been published on the oxidation of AsIII by traditional chemical oxidants such as chlorine, chlorine dioxide, chloroamine, ozone, hydrogen peroxide, permanganate and ferrate.
Chlorine and ozone react very rapidly with AsIII, while chloramine and hydrogen peroxide are sluggish in reacting. Chlorine dioxide is a powerful oxidant but it is unable to completely oxidise AsIII. Free chlorine or hypochlorite is effective for AsIII oxidation, but chlorination creates and leaves disinfectant by-products (DBPs) in treated water including trihalomethanes (THMs) which are examples of DBPs that have been shown to be carcinogenic. Ozone can reduce levels of THMs and halo acetic acids (HAAs), but it can form the potent carcinogenic bromate ion by reacting with bromide present in water.
Ferrate(VI) (FeVIO42-, FeVI) can address some of the concerns related to the use of other chemical oxidants for removing arsenic. Interestingly, FeVI does not react with bromide ions and thus carcinogenic bromate ions would not be produced in the treatment of bromide-containing water. Moreover, non-toxic FeIII is a by-product of FeVI i; this acts as a powerful coagulant that is suitable for the removal of AsV, the oxidised product of AsIII. Thus FeVI acts as multifunctional chemical: oxidant, disinfectant and coagulant. Finally, the photochemical oxidation of iAsIII using UV light irradiation has been investigated, and photocatalytic oxidation using titanium dioxide also looks promising for the oxidation of arsenite to the less toxic arsenate, followed by arsenate adsorption on to particles.
Tremendous progress has been made in understanding the mechanisms of the toxicity of arsenic species, but much remains to be learned about the mechanisms of arsenic toxicity. The biogeochemistry of As in the environment is very complex but much has been elucidated in the last few decades about As speciation and the important parameters and processes that affect the speciation and mobility of As under different conditions. While effective methods based on chemical, photochemical and photocatalytic oxidation and adsorption onto coagulants exist for removing arsenic from drinking water supplies, more information on the removal efficiency of these methods under environmentally relevant conditions is needed.