Adsorption of chromium(VI) and nickel(II) ions on acid- and heat-activateddeoiled spent bleaching clay

De-oiled spent bleaching clay was activated either by acid treatment follow ed by heat activation or by heat activation alone at temperatures between 2 00 and 800 degreesC. The surface area of the heat-activated clay attained a maximal value of similar to 120 m(2) g(-1) at temperatures between 400 and 500 degreesC while the acid-heat-treated clay attained maximal surface are a of similar to 140 m(2) g(-1). The adsorption capacities of chromium [Cr(V I)] for both series studied increased as the activation temperature increas ed until 300 degreesC and decreased again at higher temperatures. At lower pH, more than 95 % of the Cr(VI) was absorbed in a solution with initial co ncentration of 1 mg L-1 per gram of adsorbent activated at 300 degreesC. Th e adsorption patterns followed Freundlich's isotherms. Two maximal values o f adsorption capacities of nickel [Ni(II))] were observed at activation tem peratures of 200 and 500 degreesC for acid-treated samples, whereas these w ere at 200 and 700 degreesC for the nonacid-treated samples. The amount of Ni(II) adsorbed increased with the pH of the solution for all samples studi ed. The maximal adsorption capacities of the adsorbents in solution contain ing initial Ni(II) concentration of 5 mg L-1 per 0.5 g of adsorbent and at pH 6 were found to be 44 and 42%, respectively, for the acid-treated sample activated at 500 degreesC and for the nonacid-treated sample activated at 700 degreesC. They all obeyed both the Langmuir's and Freundlich's isotherm s.