Coagulation removal of humic acid-stabilized carbon nanotubes from water by PACl: influences of hydraulic condition and water chemistry

Discharged carbon nanotubes (CNTs) can adsorb the widely-distributed humic acid (HA) in aquatic environments and thus be stabilized. HA-stabilized CNTs can find their way into and challenge the potable water treatment system. This study investigated the efficiency of coagulation and sedimentation techniques in the removal of the HA-stabilized multi-walled carbon nanotubes (MWCNTs) using polyaluminum chloride (PACl) as a coagulant, with a focus on the effects of hydraulic conditions and water chemistry. Stirring speeds in the mixing and reacting stages were gradually changed to examine the effect of the hydraulic conditions on the removal rate. The stirring speed in the reacting stage affected floc formation and thereby had a greater impact on the removal rate than the stirring speed in the mixing stage. Water chemistry factors such as pH and ionic strength had a significant effect on the stability of MWCNT suspension and the removal efficiency. Low pH (4-7) was favorable for saving the coagulant and maintaining high removal efficiency. High ionic strength facilitated the destabilization of the HA-stabilized MWCNTs and thereby lowered the required PACl dosage for the coagulation. However, excessively high ionic strength (higher than the critical coagulation concentration) decreased the maximum removal rate, probably by inhibiting ionic activity of PACl hydrolyzate in water. These results are expected to shed light on the potential improvement of coagulation removal of aqueous stabilized MWCNTs in water treatment systems.