Studies on the biosorption of hexavalent chromium from aqueous solutions by using boiled mucilaginous seeds of Ocimum americanum

Abstract

Investigations were carried out to study the chromium removal efficiency of boiled mucilaginous seeds of Ocimum americanum. Batch experiments were conducted to study the biosorption kinetics of chromium removal for the concentrations 10 mg/L, 20 mg/L and 40 mg/L of chromium(VI) solutions. The biosorbent dosage was 8 g dry seeds/L. The toxic hexavalent chromium was reduced to less toxic chromium(III) in the presence of seeds and the reduced chromium was adsorbed on the mucilage of seeds. Both the chromium(VI) and chromium(III) were present in the aqueous phase. The optimum chromium reduction and adsorption was observed at the pH value 1.5. The biosorption data fitted well with Langmuir isotherm. The biosorption capacity calculated from the Langmuir isotherm was q = 32 mg chromium(III)/g of dry seeds. The continuous column study was also carried out at the flow rate of 27 mL/h for the initial concentration 25 mg/L of chromium(VI) feed solution using a packed bed column filled with boiled mucilaginous seeds. The maximum reduction of chromium(VI) to chromium(III) in the packed bed was 80%. The percentage removal of reduced chromium from the aqueous solution was 56.25%. This value was maintained constant until 0.52 L of chromium(VI) solution was pumped through the packed bed column. Thus the naturally immobilized polysaccharides on the seeds mimic the microbial polysaccharides in terms of their ability to adsorb heavy metals with an added advantage of making the immobilization step unnecessary which is a major cost factor of the metal removal process when microbial exopolysaccharides used. The uniform size and spherical shape of swollen seeds give an additional advantage to use them in a packed bed column for continuous removal of chromium(VI) from aqueous solutions.