Effects of hardness and alkalinity on the removal of arsenic(V) from humic acid-deficient and humic acid-rich groundwater by zero-valent iron

摘要

The effects of hardness (Ca~(2+)) and alkalinity (HCO_3~-) on arsenic(V) removal from humic acid (HA)-deficient and HA-rich groundwater by zero-valent iron (Fe~0) were investigated using batch experiments. Arsenic, in general, is removed from groundwater possibly by adsorption and co-precipitation with the iron corrosion products. However, in the co-presence of HCO_3~- and Ca~(2+), the removal rate of arsenic increased with increasing concentrations of either Ca~(2+) or HCO_3~-. It was observed that the removal of arsenic was significantly enhanced by the formation of CaCO_3 as a nucleation seed for the growth of large iron (hydr)oxide particles. In the co-existence of Ca~(2+), HCO_3~- and HA, the presence of HA diminished the positive role of Ca~(2+) due to the formation of Fe-humate complexes in solution and delaying of the formation of CaCO_3. As a result, the formation of the large iron (hydr)oxide particles was inhibited in the earlier stage which, in turn, affected the removal of arsenic. However, after the formation of CaCO_3 and the subsequent growth of such particles, the presence of large iron (hydr)oxide particles resulted in the rapid removing of arsenic and Fe-humate by adsorption and/or co-precipitation.