Physicochemical interaction of Escherichia coli cell envelopes and Bacillus subtilis cell walls with two clays and ability of the composite to immobilize heavy metals from solution

Isolated Escherichia coli K-12 cell envelopes or Bacillus subtilis 168 cell walls were reacted with smectite or kaolinite clay in distilled deionized water (pH 6.0); unbound envelopes or walls were separated by sucrose density gradient centrifugation, and the extent of adsorption was calculated. At saturation, both clays adsorbed approximately 1.0 mg (dry weight) of envelopes or walls per mg (dry weight) of clay. Clays showed a preference for edge-on orientation with both walls and envelopes, which was indicative of an aluminum polynuclear bridging mechanism between the wall or envelope surface and the clay edge. The addition of heavy metals increased the incidence of planar surface orientation, which suggested that multivalent metal cation bridging was coming into play and was of increasing importance. The metal-binding capacity of isolated envelopes, walls, clays, and envelope-clay or wall-clay mixtures was determined by atomic absorption spectroscopy after exposure to aqueous 5.0 mM Ag⁺, Cu²⁺, Cd²⁺, Ni²⁺, Pb²⁺, Zn²⁺, and Cr³⁺ nitrate salt solutions at pHs determined by the buffering capacity of wall, envelope, clay, or composite system. The order of metal uptake was walls > envelopes > smectite clay > kaolinite clay for the individual components, and walls plus smectite > walls plus kaolinite > envelopes plus smectite > envelopes plus kaolinite for the mixtures. On a dry-weight basis, the envelope-clay and wall-clay mixtures bound 20 to 90% less metal than equal amounts of the individual components did. This reduction in metal-binding capacity indicates that the adsorption of the wall or envelope to clay has masked or neutralized chemically reactive adsorption sites normally available to metal ions. Transmission electron microscopy and energy-dispersive X-ray spectroscopy confirmed these results and gave high spatial resolution to the regions of high binding density. For example, small electron-dense precipitates were seen associated with Ag-, Pb-, and Cr-exposed B. subtilis walls, but not with any of the composites or with E. coli envelopes.