Monothioarsenate transformation kinetics determines arsenic sequestration by sulfhydryl groups of peat

In peatlands, arsenite was reported to be effectively sequestered by sulfhydryl groups of organic matter. To which extent porewater arsenite can react with reduced sulfur to form thioarsenates and how this affects arsenic sequestration in peatlands, is unknown. Here, we show that in the arsenic-rich peatland Gola di Lago, Switzerland, up to 93% of all arsenic species in surface and porewaters were thioarsenates. The dominant species, monothioarsenate, likely formed from arsenite and surface-associated zero-valent sulfur (S(0)). Laboratory incubations with sulfide-reacted peat showed for both, monothioarsenate and arsenite, increasing total arsenic sorption with decreasing pH from 8.5 to 4.5. However, X-ray absorption spectroscopy revealed no binding of monothioarsenate via sulfhydryl groups. The sorption observed at pH 4.5 was acid-catalyzed dissociation of monothioarsenate, forming arsenite. The lower the pH and the more sulfhydryl sites, the more arsenite sorbed which in turn shifted equilibrium towards further dissociation of monothioarsenate. At pH 8.5, monothioarsenate was stable over 41 days. In conclusion, arsenic is effectively sequestered in anoxic, acidic environments where arsenite is the only arsenic species. Where fluctuating redox conditions enable sulfide oxidation to S(0), monothioarsenate forms and at neutral to alkaline pH, slow transformation kinetics make this species highly mobile.