Pyrolytic Synthesis and Characterization of Biochar Derived from Rice Husks for Removal of Selected Heavy Metals from Water

Abstract

Accumulation of heavy metals such as lead, copper, and cadmium released by various industries, poses a potential ecological risk of contamination due to their persistent nature and non-biodegradability. A remedial action was needed to mitigate this risk, adsorption of Pb2+, Cu2+and Cd2 was carried out using eco-friendly and cost-effective activated biochar from rice husk as a potential adsorbent. In order to evaluate the efficiency of rice husk biochar in the removal of lead, copper, and cadmium in aquatic systems. The rice husk (Oryza sativa) biochars were pyrolyzed at different temperatures between 300 – 700 °C in a muffle furnace. The optimal biochar was produced at 500°C, which was later activated chemically using KOH and ZnCl2. Characterization of the biochars was achieved by use of Fourier Transform Infrared, Scanning electron microscope for the surface morphology and Brunauer Emmert Teller for the surface area determination. The batch adsorption studies were conducted to determine the best conditions for effective and efficient metal ions uptake. The Infrared spectra of the rice husk biochars were characterized by O-H, -CH, C=C, and C=O functional groups, which played a key role in contributing to the adsorption process. The morphology of chemically activated optimal biochar indicated it was the best adsorbent. The Brunauer Emmert Teller (BET) results revealed that specific surface area increased with pyrolytic temperature, was the highest in RHBT600 and lowest in unpyrolyzed rice husk. The optimal contact time for adsorption for Pb2+and Cu2+ was observed to be 60 minutes while Cd2+ was 30 minutes at all initial metal ions concentrations of 1, 2, and 4 mg/L. Optimum adsorbent dosage of all three metals was found to be 0.250 g, initial metal ion concentration for Pb2+, Cu2+ and Cd2+ was 2 mg/L. The optimal pH for the three metal ions, Pb, Cu2+ and Cd2+ was (7.0) and the optimal adsorption temperature was found to be 35℃ for all metal ions. The pseudo second order was found to fit well because it had relatively high linear regression coefficient values for Pb2+ (0.995), Cu2+ (0.7906) and Cd2+ (0.999). The equilibrium fitted the Langmuir equation well, with higher linear regression coefficient values for Pb2+(0.98632) and Cu2+(0.9889) indicating monolayer adsorption on a homogeneous material. The Cd2+ metal ion removal best fitted to the Jovanovich isotherm model on a monolayer adsorption with R2 value of 0.9994. The Freundlich adsorption process was favorable for Cu2+ and Pb2+ because n>1 that is Pb2+ (2.3229) and Cu2+ (1.82615) indicating a strong bond between rice husk biochar and the two metal ions but, for Cd2+ was 0.23496 indicating normal adsorption since n<1 and linear regression coefficient values for Pb2+ (0.9675), Cu2+ (0.85181) and Cd2+ (0.39893). This indicated that Pb2+ and Cu2+ underwent both monolayer and multilayer adsorption, unlike the Cd2+ ion. The chemically activated rice husk biochar has proved to be a viable low-cost adsorbent with excellent adsorption ability in the removal of heavy metal ions.