Examination of solidified and stabilized matrices of arsenic containing sludge

Abstract

Brownish sludge as a by-product of arsenic removal by coagulation/co-precipitation with ferric chloride is in fact arsenic bonded to surface of iron hydroxide through either inner or outer-sphere surface mechanism. This research was conducted to investigate leaching behavior of arsenic from the sludge and potential of the sludge to be hazardous waste as a function of initial arsenic concentrations and finished water targets. It was found that pH of extraction fluid might play a crucial role in controlling leachability of arsenic from the sludge because it influenced arsenic desorption and resorption as well as dissolution and reformation of iron hydroxide surface. In addition, it can be concluded that the sludge produced from removal of initial arsenic concentration ranging from 140 to 12,790 ug/l to comply with the dirinking water standards of 10 and 50 ug/l of arsenic is unlikely to be hazardous waste. In contrast, the sludge from pretreatment of Ion Exchange brine to some TBLLs can be classified as hazardous waste. To reach a compromise between reasonable operation costs of waste management and sufficient long-term stability of the waste itself, this study also examined utilization potential of solidified/stabilized products. Although addition of arsenic-iron hydroxide sludge seemed to physically and chamically reduce compressive strength of the mortar due to replacement of cement, reduction of bearing by macroencapsulation of the sludge, and hydration inhibition by calcium-arsenic compounds, utilizaiton of the solidified/stabilized sludge as an interlocking concrete paving block is also possible according to TIS 827-2531 (1988). The mix proportion for this purpose is water-to-binder ratio of 0.4, waste-to-binder ratio of 0.15, and lime-to-binder ratio of 0.