Removal of Heavy Metals from Solutions Using Cajanus Cajan Husk and Butea Monosperma Leaves as Biosorbents

Abstract

In the last few decades one of the major concerns of society, industry and environmentalists is the prevention and abatement of water pollution. The presence of toxic and non-biodegradable heavy metals in industrial effluents poses extreme environmental problems as they potentially contaminate the soil and water bodies. The removal of heavy metals from wastewater has become crucial to meet safe discharge standards. Along with other commercially available technologies adsorption process is widely used in industries. Development of more economic process has been strived owing to high cost of adsorbents. Thus, biosorption process has become the area of interest to researchers and engineers. To make biosorption process commercially viable, researchers are searching for more efficient, durable and cost effective biosorbents. The present study aimed to investigate the potential use of physically treated and chemically modified Cajanus cajan (Pigeon pea) husk (CCH) and Butea monosperma (Flame of the forest) leaves powder (BMLP) as novel biosorbents in removing heavy metals like cadmium, copper and mercury from wastewater. Sophisticated instrumental analysis such as FTIR, TGA, SEM and EDX were carried out to characterize the biosorbents. FTIR spectrum has confirmed the presence of prominent functional groups such as C–H, OH, C=O, COOH and C–O–C responsible for the biosorption of metals from solutions. The TGA of the biosorbents has shown the surface water loss over large temperature range. The SEM image has indicated irregular and porous structures of the sorbents. Experimentally, isotherm and kinetic studies have been carried out for both the single metal and binary metals solutions in wastewater. In case of binary metal system, removal of metals using CCH sorbent has been carried out for both electroplating industry and laboratory prepared wastewaters. Repetitive sorption-desorption experiments were performed to examine the reusability capacity of the sorbents. Thermodynamics studies have been carried to check the feasibility of the biosorption process. Several isotherm and kinetic models have been tested to identify the best isotherm and kinetic models.Mass transfer study was performed to quantify the mass transfer resistance during the transient sorption process. Optimum design of the biosorber was also carried out to identify the best biosorption process out of two-stage and three-stage processes. Optimum pH has been identified for each metal sorption case. Those are 6.0 in the case of Cd(II) removal by CCH, 5.5 for both Cu(II) and Hg(II) removal by CCH, and 6.0 in the case of Hg(II) removal due to BMLP. The equilibrium metal uptake in general has been increased with increasing initial concentration of the metal. The selected best fit isotherm and kinetic models have been identified which varied with the type of metal and sorbent, and initial metal concentration level. An enhancement in metal binding capacities, qm using Langmuir isotherm has been observed in all the cases due to chemical activation. The order of monolayer metal uptake capacity was Hg(II) > Cd(II) > Cu(II). The required equilibrium time estimated through the kinetic study has been found less for the chemically activated than the physically activated sorbent. The estimated diffusivities of the metals obtained in the intra-particle diffusion study were laid in the range of 10-7-10-5 cm2/min. In general, an improvement in biosorption ability, qm with increase in temperature from 303.15 to 313.15 K has been observed. A decline in qm has been found in some cases due to increase of temperature to 323.15 K. The negative values of the standard Gibbs free energy, at all studied temperatures for all the biosorbents and solution concentrations except Cd(II) on CCH(N) at 100 and 150 mg/L initial metal concentration and Cu(II) on CCH(CA) at 150 mg/L initial metal concentration has signified the spontaneous biosorption of metals onto the novel biosorbents. The calculated higher activation energy E(kJ/mol) values in the range of 8-16 kJ/mol obtained from the Dubinin-Radushkevich isotherm have shown selectively favorable biosorption at higher temperature. The biosorbents could be repetitively used in heavy metal sorption in industry without compromising much the uptake ability of the biosorbents. The presence of higher resistance in the boundary layer has been found for the physically treated biosorbent as compared to chemically activated biosorbent. The use of the three-stage biosorption system has been recommended in the present study over two stage biosorption process. The biosorption of two or more metals from wastewater has been observed much more complex and competitive than single-metal biosorption. It has also been found that the biosorption of Cu(II) and Zn(II) on CCH have a antagonism effect.