Arsenic chemistry in municipal sewage sludge as affected by redox potential and pH

This laboratory experiment systematically examines As speciation and solubility in sewage sludge suspensions as affected by pH and redox potential (Eh). Under oxidizing conditions, As solubility was low with the major portion of soluble As present as organic species; dimethylarsinic acid (DMAA) was always the dominant organic species. Under moderately reducing conditions (0–100 mV), arsenite was the major inorganic As species in solution and As solubility significantly increased due to dissolution of iron oxyhydroxides. Upon reduction to −250 mV, As solubility was controlled by the formation of insoluble sulfides and as a result soluble As contents dramatically decreased as compared to levels measured at 0 mV. Speciation was drastically influenced by pH. Soluble As was maximum at a neutral pH value and decreased under both more acidic and alkaline concentrations. At pH 5.0, inorganic species were the dominant forms in solution, however, at pH 6.5, organic species were the major soluble forms. Alkaline conditions (pH 8.0) resulted in similar concentrations of soluble inorganic and organic As species. Arsenic biomethylation was measured at pH 6.5 and 8.0, but was drastically restricted at pH 5.0.