The potential of immobilied algal cell systems for the removal of mercury from aqueous solution

Abstract

The use of immobilized Chiorella ernersonii for the removal of inorganic mercury from aqueous solution in batch and continuous-flow processes was inyestigated. Initial experiments showed that HgCl2 added to growth mediun. at I mgg -I r did not markedly affect the growth of non-immobilized Chiorella at io cell ml.
Batch culture experiments using 203 Hg showed that whilst mercury was removed from medium and accumulated by alginate-immobilized and non-immobilized Clzlorella, a considerable amount of mercury was lost by volatilization. Inunobilized cells accumulated significantly more mercury than non-immobilized cells at higher inoculum densities, probably due to greater growth of immobilized cells. Living cells accumulated significantly more mercury than heat-killed cells, though mercury volatilization was greater with killed cells, indicating that the mechanism of volatilization was nonenzymic. Calcium alginate caused mercury volatilization, probably due to a chemical factor capable of transforming mercury to a more volatile form.
Continuous-flow studies using packed-bed reactors showed that unstocked agarose exhibited less mercury volatitlization than alginate. Mercury removal efficiency from medium containing 1 mg Hg r by alginate- and agarose-i;nniobilized Ciziorella
during 15-day experiments was over 90% with higher inoculum densities. Considerable cell leakage into effluent was measured. With agarose-immobilized cells, the use of high flow rates reduced mercury removal efficiency whilst lower flow rates. effluent recycling or extended operation times increased volatilization.
Scanning electron microscopy showed that cells were mainly pisent at the periphery of immobilized-cell beads in packed-bed reactors, and this may have contributed to cell leakage.
A mercury-resistant alga, isolated from the Trent-and-Mersey Canal, accumulated and volatilized mercury, suggesting that mercury volatilization may be a common mechanism of mercury resistance amongst microalgae.
Immobilized living cells were considered less suitable for mercury removal than dead cells immobilized in matrices with good mechanical strength.