Arsenic leaching characteristics of the sludges from iron based removal process

This paper examines the arsenic (As) leaching from the iron rich sludges. The sludges (S1, S2 and S3) were taken from laboratory scale As removal columns and were tested by sequential extraction procedure, toxicity characteristic leaching procedure (TCLP) and pH leaching test. The sludges were observed to have 1270 mg/kg, 705 mg/kg and 313 mg/kg of As for S1, S2 and S3, respectively. About 50–60% of the total As in the sludges was strongly bound with amorphous iron hydroxides. TCLP extraction results showed that concentrations of As leached were below the TCLP limit. At high alkaline conditions (pH 11) the dissolved As concentrations were as much as 35–45 times higher than at lower pH (pH 3) indicating that strongly bounded As would be released in the environment at high alkaline and low redox conditions. These laboratory experiments suggested maintaining non-alkaline and high redox conditions for minimum As solubility and mobilization from the sludges.