Chromium removal in tannery wastewaters “polishing” by Pinus sylvestris bark

Abstract

The possibility of using ground, dried and classified Pinus sylvestris bark for trivalent chromium removal in tannery wastewater “polishing” was studied. Tests were carried out with synthesized solutions whose chromium concentrations (5 and 20 mg/l) were similar to those found in some effluents of Portuguese tannery industries after physico-chemical treatment. Bark concentrations of 1 and 2 g/l were used (higher concentrations would have no practical value) and chromium removals ⩾ 90% were achieved in a 4.5–5.0 pH range. Agitation was accomplished by means of both air bubbling and vibrating plate. Slight differences occurred. Bark wetting was achieved either by simple bark/solution contact or vacuum application. No marked differences were observed. From an industrial point of view air bubbling agitation and simple bark/solution contact certainly look more interesting. The presence of other ions decreased the chromium removal though this decrease was never greater than about 20%. Pretreatments of bark were carried out and, in some cases, an increase of about 6–11% in the chromium sorption capacity of bark was observed.