P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 important because this will govern the response of As to mineral components, Fe oxides are probably the most changes in water chemistry. It will also determine the important adsorbents in sandy aquifers because of their modelling approach required for making predictions greater abundance and the strong binding affinity about possible future changes and for understanding Nevertheless, Al oxides can also be expected to play a past changes in As concentration significant role when present in quantity(Hingston et The importance of oxides in controlling the con- al, 1971; Manning and Goldberg, 1997b). Experience ration of As in natural waters has been appreciated from water treatment suggests that below pH 7.5, Al for a long time (Livesey and Huang, 1981; Matisoffet al, hydroxides are about as effective as Fe hydroxides(on 1982)and there has been a wide range of studies to molar basis) for adsorbing As(V) but that Fe salts ar measure the adsorption isotherms on natural and synthetic more efficient at higher pH and for adsorbing As(Ill oxide minerals and to establish the sorption processes at (Edwards, 1994) the molecular scale (Table 5). Even so, importan The interactions of as with Fe oxides have been uncertainties still remain in relation to the interactions tudied in considerable detail in the laboratory and of As(lIn) and As(V) at environmental concentrations therefore provide the best insight into the likely beha and in the presence of other interacting ions. Fre- vior of As-mineral interactions in aquifers. However, quently, the element which correlates best with As in most of these laboratory studies, particularly the older sediments is Fe. This is also the basis for the use of Fe studies have been undertaken at rather high as salts(as well as Al and Mn salts) in water treatment for centrations(Hingston et al., 1971)and there is a paucity the removal of As and other elements(e.g. Edwards, of reliable adsorption data at the low ug I-I level of 1994). The As content of residual sludges from water relevance to natural waters. In addition, there is uncer- treatment can be in the range 1000-10, 000 mg kg tainty over the extent to which the Fe oxides most (Forstner and Haase, 1998: Driehaus et al., 1998). Clays commonly studied in the laboratory reflect the Fe oxides can also adsorb As(Ill) and As(V)(Manning and found in the field. Field data for As(V)adsorption to Goldberg, 1997b) but their role in sediments in terms of natural 'diagenetic Fe oxides(captured in a lake with As binding is unclear at presen vertically-installed Teflon sheets) closely paralleled the It is difficult to study mineral-water interactions laboratory data of Pierce and Moore(1982) which was directly in aquifers. Most studies, including those with a included in the dzombak and morel (1990)data base(De bearing on As in groundwater, have been undertaken either Vitre et al., 1991). However, it was considerably greater in soils, or in lake or ocean sediments and usually from than that calculated using Hingston et al.s(1971)data quite shallow depths. There is much to be learnt from the for As(V)adsorption on goethite, highlighting the high studies of soils and sediments since the same general prin- affinity for As of freshly-formedamorphous' Fe oxides. areas where cross-fertilization of ideas can occur is in acid dissolution of a synthetic ferrihydrite containing understanding the behaviour of Fe oxides in reducing soils sorbed As(V)and concluded that the dissolution was and sediments and the influence of this on the release of as incongruent (i.e. Fe and As were not released in the Matisoff et al. ( 1982) related reductive dissolution of Fe same proportion as found in the bulk mineral) and that oxides to the possible release of As in groundwater from an the initial As released was probably sorbed on the sur- alluvial aquifer in NE Ohio Korte(1991)and Korte and face of the very small ferrihydrite particles. The same is Fernando(1991)also speculated that t desorption of As likely to happen during reductive dissolution. The from Fe oxides could occur in reducing, alluvial sediments adsorbed As also slowed down the acid dissolution of and that this could lead to high -As groundwaters. the ferrihydrite 4. 2. Arsenic associations in sediments 43. Reduced sediments and the role of iron oxides The major minerals binding As(as both arsenate and A well-known sequence of reduction reactions occurs arsenite)in sediments are the metal oxides, particularly when lakes, fjords, soils, sediments and aquifers become those of Fe, Al and Mn(De Vitre et al., 1991; Sullivan anaerobic(Berner, 1981; Stumm and Morgan, 1995; nd Aller, 1996). About 50% of the Fe in freshwater Langmuir, 1997). The processes causing changes in F sediments is in the form of Fe oxides and about 20% of redox chemistry are particularly important since they the Fe is 'reactive Fe. Clays also adsorb As because can directly affect the mobility of As. One of the princi the oxide- like character of their edges. The extent of pal causes of high As concentrations in subsurface As(V) sorption to, and coprecipitation on, carbonate waters is the reductive dissolution of hydrous Fe oxides minerals is unknown but if it behaves like phosphate, it and or the release of adsorbed or combined As. Thi is likely to be strongly retained by these minerals and sequence begins with the consumption of o, and an this may limit As concentrations in groundwaters from increase in dissolved CO2 from the decomposition of limestone aquifers (Millero et al., 2001). Of these organic matter. Next, NO decreases by reduction toimportant because this will govern the response of As to changes in water chemistry. It will also determine the modelling approach required for making predictions about possible future changes and for understanding past changes in As concentrations. The importance of oxides in controlling the con￾centration of As in natural waters has been appreciated for a long time (Livesey and Huang, 1981; Matisoff et al., 1982) and there has been a wide range of studies to measure the adsorption isotherms on natural and synthetic oxide minerals and to establish the sorption processes at the molecular scale (Table 5). Even so, important uncertainties still remain in relation to the interactions of As(III) and As(V) at environmental concentrations and in the presence of other interacting ions. Fre￾quently, the element which correlates best with As in sediments is Fe. This is also the basis for the use of Fe salts (as well as Al and Mn salts) in water treatment for the removal of As and other elements (e.g. Edwards, 1994). The As content of residual sludges from water treatment can be in the range 1000–10,000 mg kg
1 (Forstner and Haase, 1998; Driehaus et al., 1998). Clays can also adsorb As(III) and As(V) (Manning and Goldberg, 1997b) but their role in sediments in terms of As binding is unclear at present. It is difficult to study mineral-water interactions directly in aquifers. Most studies, including those with a bearing on As in groundwater, have been undertaken either in soils, or in lake or ocean sediments and usually from quite shallow depths. There is much to be learnt from the studies of soils and sediments since the same general prin￾ciples are expected to apply. One of the most important areas where cross-fertilization of ideas can occur is in understanding the behaviour of Fe oxides in reducing soils and sediments and the influence of this on the release of As. Matisoff et al. (1982) related reductive dissolution of Fe oxides to the possible release of As in groundwater from an alluvial aquifer in NE Ohio. Korte (1991) and Korte and Fernando (1991) also speculated that desorption of As from Fe oxides could occur in reducing, alluvial sediments and that this could lead to high-As groundwaters. 4.2. Arsenic associations in sediments The major minerals binding As (as both arsenate and arsenite) in sediments are the metal oxides, particularly those of Fe, Al and Mn (De Vitre et al., 1991; Sullivan and Aller, 1996). About 50% of the Fe in freshwater sediments is in the form of Fe oxides and about 20% of the Fe is ‘reactive’ Fe. Clays also adsorb As because of the oxide-like character of their edges. The extent of As(V) sorption to, and coprecipitation on, carbonate minerals is unknown but if it behaves like phosphate, it is likely to be strongly retained by these minerals and this may limit As concentrations in groundwaters from limestone aquifers (Millero et al., 2001). Of these mineral components, Fe oxides are probably the most important adsorbents in sandy aquifers because of their greater abundance and the strong binding affinity. Nevertheless, Al oxides can also be expected to play a significant role when present in quantity (Hingston et al., 1971; Manning and Goldberg, 1997b). Experience from water treatment suggests that below pH 7.5, Al hydroxides are about as effective as Fe hydroxides (on a molar basis) for adsorbing As(V) but that Fe salts are more efficient at higher pH and for adsorbing As(III) (Edwards, 1994). The interactions of As with Fe oxides have been studied in considerable detail in the laboratory and therefore provide the best insight into the likely beha￾vior of As-mineral interactions in aquifers. However, most of these laboratory studies, particularly the older studies, have been undertaken at rather high As con￾centrations (Hingston et al., 1971) and there is a paucity of reliable adsorption data at the low mg l
1 level of relevance to natural waters. In addition, there is uncer￾tainty over the extent to which the Fe oxides most commonly studied in the laboratory reflect the Fe oxides found in the field. Field data for As(V) adsorption to natural ‘diagenetic’ Fe oxides (captured in a lake with vertically-installed Teflon sheets) closely paralleled the laboratory data of Pierce and Moore (1982) which was included in the Dzombak and Morel (1990) database (De Vitre et al., 1991). However, it was considerably greater than that calculated using Hingston et al.’s (1971) data for As(V) adsorption on goethite, highlighting the high affinity for As of freshly-formed ‘amorphous’ Fe oxides. Paige et al. (1997) measured the As/Fe ratios during the acid dissolution of a synthetic ferrihydrite containing sorbed As(V) and concluded that the dissolution was incongruent (i.e. Fe and As were not released in the same proportion as found in the bulk mineral) and that the initial As released was probably sorbed on the sur￾face of the very small ferrihydrite particles. The same is likely to happen during reductive dissolution. The adsorbed As also slowed down the acid dissolution of the ferrihydrite. 4.3. Reduced sediments and the role of iron oxides A well-known sequence of reduction reactions occurs when lakes, fjords, soils, sediments and aquifers become anaerobic (Berner, 1981; Stumm and Morgan, 1995; Langmuir, 1997). The processes causing changes in Fe redox chemistry are particularly important since they can directly affect the mobility of As. One of the princi￾pal causes of high As concentrations in subsurface waters is the reductive dissolution of hydrous Fe oxides and/or the release of adsorbed or combined As. This sequence begins with the consumption of O2 and an increase in dissolved CO2 from the decomposition of organic matter. Next, NO3 - decreases by reduction to 534 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568