The effect of pH on the biosorption of Cr (III) and Cr (VI) with olive stone

In this work, the biosorption of trivalent and hexavalent chromium has been studied using olive stone, a low-cost natural sorbent, analysing the pH effect in the biosorption process. Removal of Cr (III) and Cr (VI) with olive stone is very sensitive to solution pH. The percentage of Cr (III) removed reaches a maximum at range pH between 4 and 6, reaching values close to 90%. However, at pH 3 the retention of Cr (III) is less than 50%. The percentage of Cr (VI) removed is higher than 80% when the pH is equal or lower than 2, whereas as the pH increases this percentage drops to values than 15%. Also, the results obtained for the final concentration of total Cr, Cr (VI) and Cr (III) indicate that a combined effect of biosorption and reduction is involved in the Cr (VI) removal specially when the pH value is lower than 3.