Reduction of Arsenates by Humic Materials

Abstract

Environmental Context.Reduction of arsenic(v) to arsenic(iii) in the environment is of interest because of the greater toxicity and mobility of the latter. It is important to know to what extent humic materials (which are ubiquitous in soils) can act as abiotic reducing agents, and what factors influence their actions. Abstract.Inorganic arsenates were found to be reduced to arsenite by homogeneous aqueous solutions of several humic and fulvic acids. Because of the concentration dependence of the redox potentials of humics, reduction was shown to be less likely in more concentrated solutions. This was especially true in higher pH ranges, and varied with the type of material used. Ion chromatography, validated by inductively coupled plasma mass spectrometry, was used to speciate arsenic after exposure to aqueous humates and fulvates. Reduction of As(v) proceeded in the 20–60% range, depending on the humic or fulvic acid used. The fraction of arsenate that was reduced initially increased with humic concentration, but leveled off as the reduction potential decreased at higher concentrations. Re-oxidation of As(iii) in humic solutions could be achieved by extended bubbling with air. Reduction capacities of two humates tested, as measured by I2 titration, were found to be significantly different.

Humic acids as reducing agents: the involvement of quinoid moieties in arsenate reduction

“…The pH dependence of HA redox processes has been described elsewhere (Palmer et al 2006). Humic solutions in the concentration range 10-50 µg mL −1 were prepared in N 2 -purged water with sonication and minimal addition of NaOH.…”

“…Haderlein 2003), substituted nitrobenzenes (Latifoglu and Gurol 2003), and elements such as Fe (Szilagyi 1971), Hg (Matthiessen 1998), Pu (Tan et al 1993;Andre and Choppin 2000), and As (Redman et al 2002;Bauer and Blodau 2006;Palmer et al 2006). These redox transformations have been studied both in the presence and absence of O 2 because it is known that atmospheric oxygen has an effect on the redox behavior of HA (Hayashi et al 2001;Ratasuk and Nanny 2007).…”

Arsenic in Natural Environments

“…Although humic or fulvic acids will reduce As(V) to As(III) (Palmer, Freudenthal and von Wandruszka, 2006;Tongesayi and Smart, 2006), NOM is often effective in oxidizing As(III) (Redman, Macalady andAhmann, 2002, 2895). Organic carbon in groundwater, soils, and sediments also frequently promotes the bacterial reduction of sulfate to sulfide and the formation of strong anoxic conditions.…”

Design and development of an automated flow injection instrument for the determination of arsenic species in natural waters

“…Ultimately, the concentrations and speciation of As͑III͒ and As͑V͒ are determined by various abiotic and biologically mediated biogeochemical processes. 3,[8][9][10] Traditionally, water quality was assessed by collecting aquatic samples in the field with subsequent measurement of arsenic levels by standard laboratory analytical techniques ͓e.g., atomic fluorescence spectroscopy, graphite furnace atomic absorption, and inductively coupled plasma-mass spectrometry ͑ICP-MS͔͒. 11,12 Such methods excel for routine analysis of total arsenic concentrations but fail in their attempt to measure arsenic speciation ͓e.g., As͑III͒ and As͑V͔͒.…”