Characterization and optimization of Ni and Cu sorption from aqueous solution by Chlorella vulgaris

Abstract

Sorption of Ni and Cu by Chlorella vulgaris showed the second-order rate kinetics. Change in biomass concentration altered the kinetic parameters of sorption. When biomass concentration was increased from 5 to 1000 mg l⁻¹, the initial rates of sorption of Ni and Cu were reduced by about five- and three-times, respectively. The metal sorption capacity of the test alga was studied taking different concentrations of Ni and Cu at different biomass concentrations as well as different pH. The sorption of test metals fitted better in Freundlich than the Langmuir model thereby indicating multi-layer adsorption of Ni and Cu onto C. vulgaris. The K_f and Q_max both decreased with increase in biomass concentration thereby suggesting that the metal sorption capacity of the test alga was impaired at higher biomass concentrations. The maximum sorption of Ni and Cu occurred at pH 5.5 and 3.5, respectively. Heat-killed cells showed a greater potential of metal sorption than the live cells. The test alga was subjected to different pre-treatments to enhance its metal sorption capacity; acid (HCl and HNO₃) pre-treatments were most effective. The maximal removal of Ni and Cu, 93 and 96%, respectively, occurred from solutions having their 2.5 mg l⁻¹ concentration. Thus C. vulgaris has a great potential for removing Ni and Cu especially when concentrations of these metals are low in the external environment.

Introduction

Heavy metals, one of the most toxic pollutants, often occur in industrial effluents at very high concentrations, thus posing a serious threat to biota and the environment. Therefore, it is necessary to alleviate metal load of an effluent before its discharge into a water body. Industries often employ physicochemical processes, such as precipitation (hydroxide, sulphide), ion exchange, adsorption (GAC, PAC), electrochemical treatment, solvent extraction and evaporation, for removing toxic metals from wastewater (Eccles, 1999). These processes are expensive, and also have other shortcomings, such as incomplete removal of metals, limited tolerance to pH change, moderate- or no metal selectivity, very high or low working levels of metals, and production of toxic sludge or other waste products that also need disposal (Bunke et al., 1999, Eccles, 1999). Hence, environmental and public health engineers have been searching for an inexpensive and efficient technology for the treatment of metal-containing wastes. The use of biological materials to remove metals is one of such technology that has received considerable attention in the past two decades.

Several micro-organisms, such as protozoa, bacteria, fungi and microalgae, have been tested for their ability to accumulate heavy metals (Gadd and White, 1985, Volesky and Holan, 1995, Kratochvil and Volesky, 1998, Bunke et al., 1999). The accumulation of metals by these organisms involves uptake into the intracellular compartments by an energy-dependent process, and passive attachment onto the cell surface (adsorption). However, the term sorption, encompassing all the mechanisms by which living as well as dead micro-organisms remove metal ions from external environment (Muraleedharan and Venkobachar, 1990), has been frequently used by previous researchers. Microalgae can be used as metal biosorbents in vital as well as in devitalized (dead) forms. Generally, dead algae have been shown to have a higher metal-sorption capacity (Volesky, 1990), but some workers report live cells to be more efficient for the purpose (Torres et al., 1998). For metal removal applications, the use of dead or denatured biomass may be preferred, as its large quantities are readily available. In addition, devitalized biomass is not subject to metal toxicity, so high metal concentrations are tolerated, and needs for nutrient supply and culture maintenance are obviated.

A majority of metal-biosorption studies have been conducted taking very high concentrations of sorbents and metals. Several earlier workers (Kuyucak and Volesky, 1989) have used the concentrations of metals as high as 1000 mg l⁻¹ despite the fact that such high concentrations may exist only in rare cases. Thus, it seems appropriate to carry out metal biosorption studies at lower concentrations of metals as well. Langmuir and Freundlich isotherms have often been used in metal sorption studies for interpreting the results although they do not provide information on the rate of metal uptake. It should be kept in view that information on metal uptake by a biosorbent would be indispensable for testing a technology on a pilot scale or its real world application. For instance, rapid uptake of a metal would allow a short solution–biosorbent contact time and would result in the use of much shallower contact beds of sorbing material in their column applications (Kuyucak and Volesky, 1988). Some preliminary studies have shown that various pre-treatments could enhance metal sorptive potential of algal biomass and this aspect therefore deserves adequate emphasis (Volesky and Holan, 1995).

The present study examines the feasibility of using Chlorella vulgaris, a virtually ubiquitous green alga often occurring abundantly in high-rate oxidation ponds, for removing Cu and Ni from solutions containing low concentrations of these metals. The results have been interpreted in terms of sorption isotherms as well kinetics of the sorption process. No effort was made to distinguish metal adsorbed on the surface from that inside the cell because the prime objective of the study has been to assess metal removal efficiency of the test alga. The feasibility of dead cells in removing Ni and Cu from solutions has also been tested. In addition, an evaluation of metal biosorptive potential of the test alga vis-à-vis various pre-treatments were also made.