Natural Organic Matter Affects Arsenic Speciation and Sorption onto Hematite

TLDR

Arsenic mobility in natural waters is largely governed by sorption onto metal oxide surfaces, and the presence of dissolved substances such as natural organic matter (NOM) can alter this sorption process. This study aimed to determine how different NOM samples affect arsenic sorption onto hematite and how arsenic influences NOM sorption under natural freshwater conditions. The authors examined six NOM samples, measuring arsenate and arsenite complexation, redox activity, and sorption dynamics on hematite under environmentally relevant concentrations. The results showed that NOM forms aqueous complexes with arsenate and arsenite, especially when rich in Fe, exhibits strong redox activity, delays and reduces arsenic sorption onto hematite, displaces sorbed arsenic and NOM in a competitive manner, and that arsenite is more readily desorbed or prevented from sorbing than arsenate, highlighting NOM’s significant role in arsenic mobility.

Abstract

Arsenic mobility in natural environments is controlled primarily by sorption onto metal oxide surfaces, and the extent of this sorption may be influenced strongly by the presence of other dissolved substances that interact with surfaces or with arsenic itself. Natural organic matter (NOM), a prevalent constituent of natural waters, is highly reactive toward both metals and surfaces and is thus a clear candidate to influence arsenic mobility. The objectives of this study were therefore to reveal the influences of diverse NOM samples on the sorption of arsenic onto hematite, a model metal oxide, as well as to reveal influences of arsenic on the sorption of NOM, using conditions and concentrations relevant to natural freshwater environments. Of the six NOM samples tested, four formed aqueous complexes with arsenate and arsenite. The extent of complexation varied with the NOM origin and, in particular, increased with the cationic metal (primarily Fe) content of the NOM sample. In addition, every NOM sample showed active redox behavior toward arsenic species, indicating that NOM may greatly influence redox as well as complexation speciation of arsenic in freshwater environments. When NOM and As were incubated together with hematite, NOM dramatically delayed the attainment of sorption equilibrium and diminished the extent of sorption of both arsenate and arsenite. Consistent with this result, when NOM and As were introduced sequentially, all NOM samples displaced sorbed arsenate and arsenite from hematite surfaces, and arsenic species similarly displaced sorbed NOM from hematite in significant quantities. Competition between NOM and As for sorption thus appears to be a potentially important process in natural waters, suggesting that NOM may play a greater role in arsenic mobility than previously recognized. In addition, in all sorption experiments, arsenite was consistently desorbed or prevented from sorbing to a greater extent than arsenate, indicating that interactions with NOM may also partially explain the generally greater mobility of arsenite in natural environments.