P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 concluded that the As(v)/As(Ill) ratio may be used as a rapidly being catalysed by bacteria with rate constant reliable redox indicator for groundwater syste anging from 0.02 to 0.3 day-(Oremland et aL., 2000). (1988)found that the Eh calculated from the As(V)- cally and biologically(Abdullah et al, 19%ed chemi ever, this optimism may be unfounded since Welch et al Methylated As species are also readily oxidise As(ln couple neither agreed with that from the Fe(ll) Less is known about the rate of solid-phase reductio Fe(lll and other redox couples nor with the measured of As(V) to As(lIn but there have been some studies Eh. Measurements of Eh in natural waters using Pt with soils and sediments. The evidence from soils is th electrodes are known to be problematic(Lindberg and under moderately reducing conditions(Eh< 100 mV) Runnells, 1984). The reliability of the As redox couple induced by flooding, As(V) is reduced to As(lm) in a as a redox indicator therefore remains to be seen It matter of days or weeks and adsorbed As(V) is released clearly important that where such comparisons are as As(Ill(Masscheleyn et aL., 1991; Reynolds et al made, the Eh measurements are carried out without 1999). Masscheleyn et al.(1991) found from laboratory disturbing the natural redox environment (Yan et al. experiments that some of the as was released before Fe, 2000). In cases where the aquifer is strongly stratified implying reductive desorption from Fe oxides rather groundwater flow induced by pumping during sampling than reductive dissolution. Up to 10%o of the total As in or use may also lead to the mixing of waters with very the soil eventually became soluble. Smith and Jaffe different redox potentials. Perhaps the most that can b (1998)modelled As(V) reduction in benthic sediments as said at present is that the existence of As(lIn) implies a first order reaction with respect to arsenate, with a rate reducing conditions somewhere in the system coefficient of 125 a-I Laboratory studies show that the kinetics of oxyge- nation of As(ln are slowest in the slightly acid range, around pH 5 (Eary and Schramke, 1990)which is why 3. Sources of arsenic water samples are often acidified to about this ph to preserve their in situ speciation. Eary and Schramke 3.1. Minerals ( 1990) also gave an empirical rate equation for the reaction over the pH range 8-12.5. This was based on 3.1.1. Major arsenic minerals the concentration (activity) of the H,AsOs species in Arsenic occurs as a major constituent in more than solution. They suggested that the half-life for As(ln) in 200 minerals, including elemental As, arsenides, sul natural waters is 1-3 a, although the rate may be greater hides, oxides, arsenates and arsenites. A list of some of because of the presence of unknown aqueous species or the most common As minerals is given in Table 2. Most oxide particles, especially Mn oxides. Certainly there is are ore minerals or their alteration products. However, considerable evidence that mn oxides can increase the these minerals clatively rare in the natural environ- rate of As(ln oxidation with half-lives being reduced to nent. The greatest concentrations of these minerals as little as 10-20 min in the presence of Mn-oxide par- occur in mineralised areas and are found in close asso- ticles(Oscarson et al., 1981; Scott and morgan, 1995) ciation with the transition metals as well as Cd, Pb, ag, This is used to advantage in the removal of As(lln) from Au, Sb, P, w and Mo. The most abundant As ore drinking water(Driehaus et aL., 1995). The rate of oxida- mineral is arsenopyrite, FeAss. It is generally believed ion is independent of the concentration of dissolved O2 that arsenopyrite, together with the other dominant As (Scott and Morgan, 1995), the rate being controlled by the sulphide minerals realgar and orpiment, are only formed te of a surface reaction. Less is known about the role of under high temperature conditions in the earth's crust Fe oxides in altering the oxygenation kinetics. Phote However, authigenic arsenopyrite has been reported in chemical oxidation and reduction may be additional sediments by Rittle et al.(1995)and orpiment has factors in surface waters. Titanium-containing particles recently been reported to have been formed by may aid the photo-oxidation(Foster et al., 1998) bial precipitation(Newman et al., 1998). Although often In the natural environment, the rates of both As(ll) present in ore deposits, arsenopyrite is much less abun- xidation and As(V) reduction reactions are controlled dant than arsenian (As-rich") pyrite(Fe(s, As)) which isms and can be orders of magnitude is probably the most important source of As in or greater than under abiotic conditions. For example, zones(Nordstrom, 2000) sterile water samples have been observed to be less sus- Where arsenopyrite is present in sulphide ores asso- eptible to speciation changes than non-sterile ciated with sediment-hosted Au deposits, it tends to be (Cullen and reimer, 1989). Wilkie and Hering (1998) the earliest-formed mineral, derived from hydrothermal found that As(m in geothermal waters input to streams solutions and formed at temperatures typically of 100C in Sw USA oxidised rapidly downstream(pseudo first- or more. This is followed by the formation of rarer order half-life calculated at as little as 0.3 h) and they native As and thereafter arsenian pyrite. Realgar an attributed the fast rate to bacterial mediation. The orpiment generally form later still. This paragenetic duction of As(v) to As(Im in Mono Lake was also sequence is often refected by zonation within sulphidconcluded that the As(V)/As(III) ratio may be used as a reliable redox indicator for groundwater systems. How￾ever, this optimism may be unfounded since Welch et al. (1988) found that the Eh calculated from the As(V)– As(III) couple neither agreed with that from the Fe(II)– Fe(III) and other redox couples nor with the measured Eh. Measurements of Eh in natural waters using Pt electrodes are known to be problematic (Lindberg and Runnells, 1984). The reliability of the As redox couple as a redox indicator therefore remains to be seen. It is clearly important that where such comparisons are made, the Eh measurements are carried out without disturbing the natural redox environment (Yan et al., 2000). In cases where the aquifer is strongly stratified, groundwater flow induced by pumping during sampling or use may also lead to the mixing of waters with very different redox potentials. Perhaps the most that can be said at present is that the existence of As(III) implies reducing conditions somewhere in the system. Laboratory studies show that the kinetics of oxyge￾nation of As(III) are slowest in the slightly acid range, around pH 5 (Eary and Schramke, 1990) which is why water samples are often acidified to about this pH to preserve their in situ speciation. Eary and Schramke (1990) also gave an empirical rate equation for the reaction over the pH range 8–12.5. This was based on the concentration (activity) of the H2AsO3 - species in solution. They suggested that the half-life for As(III) in natural waters is 1–3 a, although the rate may be greater because of the presence of ‘unknown aqueous species’ or oxide particles, especially Mn oxides. Certainly there is considerable evidence that Mn oxides can increase the rate of As(III) oxidation with half-lives being reduced to as little as 10–20 min in the presence of Mn-oxide par￾ticles (Oscarson et al., 1981; Scott and Morgan, 1995). This is used to advantage in the removal of As(III) from drinking water (Driehaus et al., 1995). The rate of oxida￾tion is independent of the concentration of dissolved O2 (Scott and Morgan, 1995), the rate being controlled by the rate of a surface reaction. Less is known about the role of Fe oxides in altering the oxygenation kinetics. Photo￾chemical oxidation and reduction may be additional factors in surface waters. Titanium-containing particles may aid the photo-oxidation (Foster et al., 1998). In the natural environment, the rates of both As(III) oxidation and As(V) reduction reactions are controlled by micro-organisms and can be orders of magnitude greater than under abiotic conditions. For example, sterile water samples have been observed to be less sus￾ceptible to speciation changes than non-sterile samples (Cullen and Reimer, 1989). Wilkie and Hering (1998) found that As(III) in geothermal waters input to streams in SW USA oxidised rapidly downstream (pseudo first￾order half-life calculated at as little as 0.3 h) and they attributed the fast rate to bacterial mediation. The reduction of As(V) to As(III) in Mono Lake was also rapidly being catalysed by bacteria with rate constants ranging from 0.02 to 0.3 day
1 (Oremland et al., 2000). Methylated As species are also readily oxidised chemi￾cally and biologically (Abdullah et al., 1995). Less is known about the rate of solid-phase reduction of As(V) to As(III) but there have been some studies with soils and sediments. The evidence from soils is that under moderately reducing conditions (Eh<100 mV) induced by flooding, As(V) is reduced to As(III) in a matter of days or weeks and adsorbed As(V) is released as As(III) (Masscheleyn et al., 1991; Reynolds et al., 1999). Masscheleyn et al. (1991) found from laboratory experiments that some of the As was released before Fe, implying reductive desorption from Fe oxides rather than reductive dissolution. Up to 10% of the total As in the soil eventually became soluble. Smith and Jaffe´ (1998) modelled As(V) reduction in benthic sediments as a first order reaction with respect to arsenate, with a rate coefficient of 125 a
1 . 3. Sources of arsenic 3.1. Minerals 3.1.1. Major arsenic minerals Arsenic occurs as a major constituent in more than 200 minerals, including elemental As, arsenides, sul￾phides, oxides, arsenates and arsenites. A list of some of the most common As minerals is given in Table 2. Most are ore minerals or their alteration products. However, these minerals are relatively rare in the natural environ￾ment. The greatest concentrations of these minerals occur in mineralised areas and are found in close asso￾ciation with the transition metals as well as Cd, Pb, Ag, Au, Sb, P, W and Mo. The most abundant As ore mineral is arsenopyrite, FeAsS. It is generally believed that arsenopyrite, together with the other dominant As￾sulphide minerals realgar and orpiment, are only formed under high temperature conditions in the earth’s crust. However, authigenic arsenopyrite has been reported in sediments by Rittle et al. (1995) and orpiment has recently been reported to have been formed by micro￾bial precipitation (Newman et al., 1998). Although often present in ore deposits, arsenopyrite is much less abun￾dant than arsenian (‘As-rich’) pyrite (Fe(S,As)2) which is probably the most important source of As in ore zones (Nordstrom, 2000). Where arsenopyrite is present in sulphide ores asso￾ciated with sediment-hosted Au deposits, it tends to be the earliest-formed mineral, derived from hydrothermal solutions and formed at temperatures typically of 100 C or more. This is followed by the formation of rarer native As and thereafter arsenian pyrite. Realgar and orpiment generally form later still. This paragenetic sequence is often reflected by zonation within sulphide 528 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568