Zeta potential measurements of silica-induced protein (SIP) Escherichia coli and quartz showed that the former are positively charged under acidic condition and negatively charged under neutral and alkaline conditions, with an isoelectric point (IEP) at pH 4.5, while the quartz was always negatively charged. Adsorption experiments with bacteria cells on quartz were conducted under different conditions. The results show that at pH values lower than the IEP of the cells, more cells were adsorbed due to electrostatic forces. However, at pH > 4.5, the amount of adsorbed cells decreased as a result of electrostatic repulsion forces. Zeta potentials of quartz showed that at pH 2.5 a significant change in the surface chemistry of quartz occurred after bacterial treatment. The degree of this change was related to the initial SIP E. coli concentration; at 5 x 10(7) cells/ml the average zeta potential of biotreated quartz shifted from -30 mV to 0 mV and at higher concentrations the zeta potential shifted to the positive direction and reached a similar value to that of the bacterial cells. SIP E. coli showed hydrophobic properties at pH lower than the IEP, with approximately 60% of the cells moving to the organic phase from aqueous phase. Bioflotation of quartz using SIP E coli alone at pH 2.5 gave approximately 60% recovery because at this pH more bacteria adsorb onto the quartz surface and the bacterial surface is hydrophobic. In anionic flotation of quartz using sodium dodecyl sulfate, SIP E. coli cells act as a surface modifier, with an increase in flotation recovery from 28% to 85%. This is because the bacterial cells confer hydrophobic properties to the quartz and the biotreated quartz is positively charged, so a large amount of the collector was adsorbed and the recovery increased. (C) 2007 Elsevier Ltd. All rights reserved.