Polyaluminum chloride with high Al30 content as removal agent for arsenic-contaminated well water

Polyaluminum chloride (PACl) is a well-established coagulant in water treatment with high removal efficiency for arsenic. A high content of Al(30) nanoclusters in PACl improves the removal efficiency over broader dosage and pH range. In this study we tested PACl with 75% Al(30) nanoclusters (PACl(Al30)) for the treatment of arsenic-contaminated well water by laboratory batch experiments and field application in the geothermal area of Chalkidiki, Greece, and in the Pannonian Basin, Romania. The treatment efficiency was studied as a function of dosage and the nanoclusters' protonation degree. Acid-base titration revealed increasing deprotonation of PACl(Al30) from pH 4.7 to the point of zero charge at pH 6.7. The most efficient removal of As(III) and As(V) coincided with optimal aggregation of the Al nanoclusters at pH 7-8, a common pH range for groundwater. The application of PACl(Al30) with an Al(tot) concentration of 1-5mM in laboratory batch experiments successfully lowered dissolved As(V) concentrations from 20 to 230 μg/L to less than 5 μg/L. Field tests confirmed laboratory results, and showed that the WHO threshold value of 10 μg/L was only slightly exceeded (10.8 μg/L) at initial concentrations as high as 2300 μg/L As(V). However, As(III) removal was less efficient (<40%), therefore oxidation will be crucial before coagulation with PACl(Al30). The presence of silica in the well water improved As(III) removal by typically 10%. This study revealed that the Al(30) nanoclusters are most efficient for the removal of As(V) from water resources at near-neutral pH.