Heavy metal and toxic metalloid removals from waste streams by biofilm populations

Carole Lynn Engelder, Purdue University

Abstract

A four stage continuous-flow bench-scale rotating biological contactor (RBC) was operated to maintain a constant-culture of biofilm for use in a separate batch-mode reactor. Removable discs allowed the biofilm to be transferred, at steady-state, from the continuous-flow RBC to the batch tank for studies pertaining to metals removals. Two heavy metals (cadmium and nickel) and two metalloids (arsenic and selenium) were tested in order to develop fundamental parameters of metals removals by the biofilm, including: (1) percent metals removals, (2) first-order batch metals removals kinetics, and (3) accumulation of metals within the biofilm. Variables included in the parameter development were stage origin of biofilm and presence of organic substrate. A general trend was observed for the biofilm's affinity for the four metals: Cd $>$ Ni, Se $>$ As. Biofilm efficiency, as measured by percent removal of the four metals, was found to be:(UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\vbox{\halign{\hfil#\hfil&&\enspace\hfil#\hfil\cr Cd& $>$& Ni& $>$& Se& $>$& As\cr 81.7\%& $>$ 42.8--60.6\%& $>$& 35.7\%& $>$& 17.7\%\cr}}$$(TABLE/EQUATION ENDS) Removal rates of the metals, in terms of batch kinetic constants (units, L/g hr):(UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\vbox{\halign{\hfil#\hfil&&\enspace\hfil#\hfil\cr Cd& $>$& Se& $>$& Ni& $>$& As\cr 0.052--0.103& $>$& 0.033& $>$& 0.024& $>$& 0.010\cr}}$$(TABLE/EQUATION ENDS) Biofilm inhibition was noted at heavy metal concentrations of 5 ppm. Removal of nickel decreased by 30%, while removal rate of cadmium dropped 50%. Cyanide inhibition of nickel and cadmium removals was also investigated. The biofilm was fractionated into exocellular and intracellular components by an exocellular polymer extraction procedure. The vast majority of metals accumulated intracellularly, indicating the removal mechanism may have been biological uptake, rather than physical adsorption. From an initial concentration of 5 ppm, cadmium and nickel total metal biofilm accumulation was 5.7 ug/mg. Exocellular slime levels reached a maximum of 0.9 mg metal/mg dry solids. Mass balances also indicated a loss of metals by volatilization, with arsenic losses of 13% and selenium losses up to 25%.

Degree

Ph.D.

Advisors

Alleman, Purdue University.

Subject Area

Civil engineering

COinS