Biosorbents and chemically modified biosorbents were prepared and compared with commercial ion-exchange resins to determine their metal-binding capacity, the range of metals bound, the effects of pH, temperature, contact time, interference by common salts and the effect of multiple cycles of metal binding and elution. Biosorbents were prepared from microorganisms isolated from pristine and metal impacted (acid mine drainage) environments and included heterotrophs, methanotrophs, algae, sulfate reducers, and exopolysaccharide-producing cultures. The chemical modifications examined included encapsulation in polysulfone resin, acid, alkali, carbon disulfide, phosphorus oxychloride, anhydrous formamide, sodium thiosulfate, sodium chloroacetic acid, and phenysulfonate treatments. A culture isolated from an acid mine drainage impacted sire, IGTM17, produced biosorbent material with about three-fold higher metal-binding capacity than other biosorbents examined in this study. Encapsulation in polysulfone resin of whole cultures of exopolysaccharide-producing cultures was shown to be effective in producing biosorbent, whereas the encapsulation of purified exopolysaccharide was unsuccessful. Treatments with sodium chloroacetic acid, carbon disulfide, phosphorus oxychloride, and sodium thiosulfate resulted in biosorbents with as much as 33, 74, 133 and 155% improvements in metal-binding capacity, respectively; however the performance of most chemically treated biosorbents deteriorated ripen repeated use (multiple cycles of metal binding and elution). Some chemical treatments produced biosorbents capable of binding anions. The binding of Au+3 Cd+2, Co+2 Cr+3, Cu+2, Hg+2, Ni+2, Pb+2, Se+2, Sr+2, V+2, Zn+2, AsO2-1, CrO4-2, MoO4-2, and WO4-2 was investigated. gated. All of these ions could be removed from aqueous solutions by biosorbents under some conditions and the selective sorption of particular cations or anions from mixtures of ions by some biosorbents was observed. The elution of metals bound to biosorbents was investigated wing various concentrations of hydrochloric acid, thiourea, citrate, ammonia, nitrilotriacetic acid, and ethylenediamine tetraacetic acid. Selective elution conditions were demonstrated to allow the recovery of individual metals from mixtures. Copyright (C) 1996 Elsevier Science Ltd.