Removal of aqueous Hg(II) by polyaniline: sorption characteristics and mechanisms

A polyaniline (PAN) prepared by chemical oxidation method was studied for Hg(II) removal from aqueous solutions. Batch adsorption results showed solution pH values had a major impact on mercury adsorption by this sorbent with optimal removal observed around pH 4-6. At both acidic and alkaline solutions beyond this optimal pH window, sorption capacity of PAN was substantially lowered, with the impact less pronounced at pH above 6. Among the water constituents tested, only chloride and humic acid had significant inhibition on mercury removal due to competitive complexation. In the range of 0.02-0.2 M, ionic strength had less impact on Hg(II) removal by PAN while further increase in background electrolyte concentration to 1.0 M substantially decreased mercury removal. An adsorption mechanism was proposed by analyzing the XPS spectra of the key elements (N(1s), Cl(2p) and Hg(4f)) on polyaniline surfaces and the change of its electrokinetic properties, both before and after Hg(II) adsorption. Specifically, at pH 5.5, it is likely that all the nitrogen-containing functional groups on the polymer matrix including imine, protonated imine and amine could be responsible for mercury adsorption, with imine having the highest affinity while the remaining two having similar strength to complex mercury.