The characterization and flocculation mechanism of a bioflocculant produced by Arthrobacter sp. B4 were investigated. The bioflocculant's active ingredient was a polysaccharide (B4-PS) that consisted of three main fractions corresponding to the molecular weights of approximately 3.97 x 10(4) Da, 6.84 x 10(3) Da and 5.9 x 10(6) Da, respectively. These fractions were composed of galactose, glucose, mannose and glucuronic acid. Flocculation experiments showed that B4-PS could spontaneously flocculate in the presence of Ca2+ ions at a high pH (> 12.0), followed by the pH reduction to similar to 6.0. The self-flocculation of B4-PS may be mediated by ionization and charge neutralization mechanism. Furthermore, B4-PS exhibited excellent capabilities for pollutant removal and pH reduction in alkaline wastewater. These data suggest B4-PS may be a promising tool for use in industrial alkaline wastewater pretreatment. (C) 2014 Elsevier Ltd. All rights reserved.