Bohumil Volesky

BIOSORPTION OF HEAVY METALS

Only within the past decade has the potential of metal biosorption by biomass materials been well established. For economic reasons, of particular interest are abundant biomass types generated as a waste byproduct of large‐scale industrial fermentations or certain metal‐binding algae found in large quantities in the sea. These biomass types serve as a basis for newly developed metal biosorption processes foreseen particularly as a very competitive means for the detoxification of metal‐bearing industrial effluents. The assessment of the metal‐binding capacity of some new biosorbents is discussed. Lead and cadmium, for instance, have been effectively removed from very dilute solutions by the dried biomass of some ubiquitous species of brown marine algae such as Ascophyllum and Sargassum, which accumulate more than 30% of biomass dry weight in the metal. Mycelia of the industrial steroid‐transforming fungi Rhizopus and Absidia are excellent biosorbents for lead, cadmium, copper, zinc, and uranium and also bind other heavy metals up to 25% of the biomass dry weight. Biosorption isotherm curves, derived from equilibrium batch sorption experiments, are used in the evaluation of metal uptake by different biosorbents. Further studies are focusing on the assessment of biosorbent performance in dynamic continuous‐flow sorption systems. In the course of this work, new methodologies are being developed that are aimed at mathematical modeling of biosorption systems and their effective optimization. Elucidation of mechanisms active in metal biosorption is essential for successful exploitation of the phenomenon and for regeneration of biosorbent materials in multiple reuse cycles. The complex nature of biosorbent materials makes this task particularly challenging. Discussion focuses on the composition of marine algae polysaccharide structures, which seem instrumental in metal uptake and binding. The state of the art in the field of biosorption is reviewed in this article, with many references to recent reviews and key individual contributions.

A REVIEW OF THE BIOCHEMISTRY OF HEAVY METAL BIOSORPTION BY BROWN ALGAE

The passive removal of toxic heavy metals such as Cd(2+), Cu(2+), Zn(2+), Pb(2+), Cr(3+), and Hg(2+) by inexpensive biomaterials, termed biosorption, requires that the substrate displays high metal uptake and selectivity, as well as suitable mechanical properties for applied remediation scenarios. In recent years, many low-cost sorbents have been investigated, but the brown algae have since proven to be the most effective and promising substrates. It is their basic biochemical constitution that is responsible for this enhanced performance among biomaterials. More specifically, it is the properties of cell wall constituents, such as alginate and fucoidan, which are chiefly responsible for heavy metal chelation. In this comprehensive review, the emphasis is on outlining the biochemical properties of the brown algae that set them apart from other algal biosorbents. A detailed description of the macromolecular conformation of the alginate biopolymer is offered in order to explain the heavy metal selectivity displayed by the brown algae. The role of cellular structure, storage polysaccharides, cell wall and extracellular polysaccharides is evaluated in terms of their potential for metal sequestration. Binding mechanisms are discussed, including the key functional groups involved and the ion-exchange process. Quantification of metal-biomass interactions is fundamental to the evaluation of potential implementation strategies, hence sorption isotherms, ion-exchange constants, as well as models used to characterize algal biosorption are reviewed. The sorption behavior (i.e., capacity, affinity) of brown algae with various heavy metals is summarized and their relative performance is evaluated.

Detoxification of metal-bearing effluents: biosorption for the next century

Metals can be removed and concentrated from solutions by using biomass material. Conservative estimates give new biosorbents the potential share amounting to US

ADVANCES IN THE BIOSORPTION OF HEAVY METALS

27 million/year of the currently existing environmental market in North America alone. Very high cost-effectiveness of biosorption technology would tend to open new opportunities currently untapped. Biosorbents can be regenerated for multiple reuse, offering the metal recovery possibility from concentrated wash solutions. Relatively simple metal biosorption processes can meet the progressively stricter environmental discharge criteria. As with any up-start technology, the continuing R&D is crucial. The interdisciplinary nature on both sides, application as well as R&D, poses quite a challenge. While there are numerous potential industrial clients, a successful biosorption enterprise will have to have courage, multidisciplinary skills and adequate financing.

BIOSORBENTS FOR RECOVERY OF METALS FROM INDUSTRIAL SOLUTIONS

The biosorption of heavy metals by certain types of non-living biomass is a highly cost-effective new alternative for the decontamination of metal-containing effluents. Our understanding of the mechanisms of metal biosorption now allows the process to be scaled up and used in field applications, with packed-bed sorption columns being perhaps the most efficient for this purpose. Regenerating the biosorbents increases the process economy by allowing their reuse in multiple sorption cycles. The process results in metal-free effluents and small volumes of solutions containing concentrated metals, which can be easily recovered.

Biosorption of heavy metals by Saccharomyces cerevisiae

SummaryBiosorbent materials are a potential alternative to conventional processes of metal recovery from industrial solutions. Algal biomass ofSargassum natans andAscophyllum nodosum outperformed ion exchange resins in sequestering respectively gold and cobalt from solutions. Non-living biomass ofSaccharomyces cerevisiae andRhizopus arrhizus exhibited higher metal-uptake capacity than the living biomass for the uptake of copper, zinc, cadmium, uranium. The solution pH affected the metal-uptake capacity of the biomass whereas the equilibrium biosorption isotherms were independent of the initial concentration of the metal in the solution. Desorption of the metal from the biosorbent and recycle of the biosorbent have also been demonstrated.

Modeling of the Proton-Metal Ion Exchange in Biosorption

Abundant and common yeast biomass has been examined for its capacity to sequester heavy metals from dilute aqueous solutions. Live and non-living biomass of Saccharomyces cerevisiae differs in the uptake of uranium, zinc and copper at the optimum pH 4–5. Culture growth conditions can influence the biosorbent metal uptake capacity which normally was: living and non-living brewers yeast: U > Zn > Cd > Cu; non-living bakers yeast: Zn > (Cd) > U > Cu; living bakers yeast: Zn > Cu ≈ (Cd) > U. Non-living brewers yeast biomass accumulated 0.58 mmol U/g. The best biosorbent of zinc was non-living bakers yeast ( ≈ 0.56 mmol Zn/g). Dead cells of S. cerevisiae removed approximately 40% more uranium or zinc than the corresponding live cultures. Biosorption of uranium by S. cerevisiae was a rapid process reaching 60% of the final uptake value within the first 15 min of contact. Its deposition differing from that of other heavy metals more associated with the cell wall, uranium was deposited as fine-needle-like crystals both on the inside and outside of the S. cerevisiae cells.

Biosorption of metals in brown seaweed biomass

Biosorption of the heavy metal ions Cd 2+ , Cu 2+ , and Zn 2+ by previously protonated nonliving biomass of the marine alga Sargassum fluitans was observed to be coupled with a release of protons. Metal ion binding experiments with continuously controlled pH were performed. The metal ion and proton binding at equilibrium were modeled as a function of pH and metal ion concentration using a modified multicomponent Langmuir sorption model. Both the exchange of metal ions for protons from functional groups in their acidic form and the sorption of metal ions on ionized groups were considered. The model is applicable to adsorption by biomass with free or protonated metal binding sites as well as to metal ion desorption with acids since the direction of the reaction depends simply on the given initial conditions. The model parameters were incorporated into the MINEQL+ equilibrium program, leading to a prediction of the equilibrium, e.g., of metal ion laden biosorbent desorption performance for given initial conditions.

Biosorbents for metal recovery

Biosorption of Cd by biomass of the brown seaweeds Durvillaea, Laminaria, Ecklonia and Homosira presaturated with Ca, Mg or K was coupled with the release of these light ions. The feasibility of biomass pre-treatment to develop a better biosorbent was evaluated by its biosorption performance, the degree of its component leaching (measured by the weight loss and TOC) as well as by the number of ion-exchange sites remaining in the biomass after the pre-treatment. Multicomponent Langmuir and ion exchange models applied to the equilibrium sorption data for pH 4.5 confirmed the ion exchange mechanism involved in the biosorption of metals. Both models fitted well the experimental data and their parameters can be used in the derivation of dimensionless ion-exchange isotherms which are instrumental in predicting the behavior of the biosorbents in dynamic flow-through biosorption systems. The sequence of biomass affinities established for the selected heavy metals can be correlated with the chemical pretreatment of the biomass.

Biosorption process simulation tools

Inactivated, non-living microbial biomass can serve as a basis for development of potent biosorbent materials for concentration and recovery of strategic or valuable heavy metals, nuclear fuel or radioactive elements. New biosorbents can be regenerated for multiple reuse. They can be highly selective, efficient and cheap, competing with commercial ion exchange resins and activated carbons in a process arrangement almost identical to that used for these conventional materials. Biosorbents have a potential application in both environmental control and metal recovery operations.