The fate of arsenic in the aquatic environment is influenced by dissolved natural organic matter (DOM). Using an equilibrium dialysis method, conditional distribution coefficients (Dom) for As(III) and As(V) binding onto two commercial humic acids were determined at environmentally relevant As/dissolved organic carbon (DOC) ratios and as a function of pH. At all pH values, As(V) was more strongly bound than As(III). Maximum binding was observed around pH 7, which is consistent with H+ competition for binding sites at low pH values and OH- competition for the arsenic center at high pH. For both oxidation states, Dom values increased with decreasing As/DOC ratios. Dom values were fitted as a function of the As/DOC ratio for As(III) and As(V). Compared to the aquatic humic acid, the terrestrial humic acid had a higher affinity for arsenic binding with 1.5-3 times higher Dom values under the same conditions. Al3+ in excess to arsenic successfully competed for strong binding sites at low As/DOC ratios. Under environmentally relevant conditions, about 10% of total As(V) may be bound to DOM, whereas >10% of As(III) is bound to DOM at very low As/DOC ratios only. Binding of arsenic to DOM should be considered in natural systems.

No abstract

Arsenic contamination of groundwater has been identified in Cambodia, where some 100,000 family-based wells are used for drinking water needs. We conducted a comprehensive groundwater survey in the Mekong River floodplain, comprising an area of 3700 km(2) (131 samples, 30 parameters). Seasonal fluctuations were also studied. Arsenic ranged from 1 to 1340 microg L(-1) (average 163 microg L(-1)), with 48% exceeding 10 microg L(-1). Elevated manganese levels (57% >0.4 mg L(-1)) are posing an additional health threat to the 1.2 million people living in this area. With 350 people km(-2) potentially exposed to chronic arsenic poisoning, the magnitude is similar to that of Bangladesh (200 km(-2)). Elevated arsenic levels are sharply restricted to the Bassac and Mekong River banks and the alluvium braided by these rivers (Kandal Province). Arsenic in this province averaged 233 microg L(-1) (median 100 microg L(-1)), while concentrations to the west and east of the rivers were <10 microg L(-1). Arsenic release from Holocene sediments between the rivers is most likely caused by reductive dissolution of metal oxides. Regions exhibiting low and elevated arsenic levels are co-incident with the present low relief topography featuring gently increasing elevation to the west and east of a shallow valley-understood as a relict of pre-Holocene topography. The full georeferenced database of groundwater analysis is provided as Supporting Information.

Reduction rate constants of seventeen polyhalogenated methanes and ethanes were measured in aqueous solutions containing bulk reductants and the electron-transfer mediators iron porphyrin or juglone (5-hydroxy-1,4naphthoquinone). Rate constants varied 4 orders of magnitude in the case of reaction with iron porphyrin and 7 orders of magnitude in the reaction with mercaptojuglone, an addition product of juglone and hydrogen sulfide. For the iron porphyrin system the results support a reaction mechanism in which one electron is transferred in an outersphere process to the polyhalogenated alkane with bond breakage in the transition state. In the juglone system two competing reaction mechanisms are proposed: an outersphere electron transfer to the polyhalogenated alkane and an S N 2 reaction at the halogen. The kinetic results are directly applicable to environmental systems containing common reductants such as natural organic matter and reduced iron species, providing a "fingerprint" of reactivity for examination of reactive species and rate-limiting steps in those systems.

Conditional distribution coefficients (Dom) for Sb(III) binding to three commercial humic acids (terrestrial, coal, and aquatic) were measured at environmentally relevant Sb(III)/DOC ratios and as a function of pH using an equilibrium dialysis method. Maximum binding of Sb(III) was observed around pH 6 for two of the humic acids. The third humic acid showed constant Dom values up to pH 6 and decreasing Dom values for pH > 6. Sb(III)/DOC ratio was found to be important for Dom (20 times higher Dom for 60 times lower Sb(III)/DOC ratio). Moreover, Dom depends on the individual humic acid, suggesting that different functional groups are involved and/or different degrees of stabilization by chelation or H-bridges. Chemical modeling of Sb(III)-humics binding at different pH values is consistent with two binding sites involving (i) a phenolic entity forming a neutral complex and (ii) a carboxylic entity forming a negatively charged complex. Under environmentally relevant conditions, over 30% of total Sb(III) may be bound to natural organic matter.