Investigation of locust bean husk char adsorbability in heavy metal removal

Abstract

Ajayi-Banji A., Ogunlela A., Ogunwande G. (2017): Investigation of locust bean husk char adsorbability in heavy metal removal. Res Agr. Eng., 63: 29-35.The column adsorption study examines irrigation water treatment prior to its application in order to ensure watercrop-consumer heavy metal transfer reduction to the safe level using locust bean husk char (LBHC) as biosorbent. Char structural pattern was investigated with SEM-EDX machine. Contaminated surface water was introduced simultaneously into the bioreactors containing 100 and 200 g of LBHC and collected after 30, 60, 90 120 and 150 min of detention time. Removal efficiency, isotherm and kinetic sorption model were the evaluation tools for the study. Percent of Cr, Cd and As removal at 150 min retention time were 83.33, 100 and 100%, respectively for 100 g biosorbents. A similar trend was observed for Cr and As removal at the same retention time for 200 g of LBHC. Metals sorption conforms to the Freundlich isotherm with correlation coefficient values greater than 0.92. Experimental kinetics had a good fit for pseudo second order (R 2 > 0.94 for most cases). Removal efficiency is a function of contact time, biosorbent dosage and metal concerned. Locust bean husk char has good and effective treatability for some heavy metals in mildly polluted water.Keywords: locust bean husk char; biosorption; heavy metal; removal efficiency; isotherm, kinetic sorption 29Res. Agr. Eng. Vol. 63, 2017 (1): 29-35 MATERIAL AND METHODSLocust bean husk was sourced locally from farmers after harvest in Ilorin, Kwara State of Nigeria (Fig. 1). The precursor (husk) was cleaned from external materials such as sand and other aggregates and weighed (Fig. 1b). Weighed locust bean husks were wrapped with aluminium foil to ensure complete deoxygenated condition during carbonization. The wrapped husks were subjected to varying carbonization temperatures and times in a pre-set muffle furnace (Model SXL) to determine the suitable carbonization temperature and time based on char quality and energy expended. The temperatures were 300°C, 350°C, at 30 and 60 minutes. Pyrolyzed precursor (Fig. 1c) was crushed into granular particle and washed thoroughly in distilled water to prevent crosscontamination. The wet char was then oven dried at 105°C for 6 h, allowed to cool and sorted with 1,400 micron sieve before storage. Locust bean husk structural pattern was examined before and after carbonization using the SEM-EDX machine (TESCAN, South Africa). Two adsorbers were underlaid with 50 g of cotton wool, filled with 100 and 200 g of the non-activated char, respectively, before being overlaid with another 50 g of cotton wool for the experimental set-up. Effluent was collected under stringent condition from a flowing stream channelled through Pipeline, Ilorin-South Local Government, Ilorin, Kwara State. The water source is used by local farmers and horticulturists for irrigation of crops such as plantain, banana, maize, coconut, vegetable and flowers. Eighteen litres of the surface water wa...