Preparation and Evaluation of Activated Carbons from Rice Husks in Uganda for Removal of Humic Acid from Water.

Abstract

The Government of Uganda and its development partners have made great strides in promoting rice production to improve household food security and income. However, the amount of rice husks (RH) generated from the milling process are usually far in excess of any immediate local application, and are usually disposed off by open dumping and/or burning, causing environmental problems. These problems could be avoided by valorizing RH into activated carbon (AC), for removal of humic acid from water, which otherwise reduces water quality, promotes bacterial regrowth, and leads to formation of disinfection by-products harmful to human health. Despite these possibilities, i) no evidence of successful utilization of RH in Uganda for preparation of AC had been reported, and ii) information regarding the performance of the rice husk-derived AC towards humic acid removal from water was also limited. This study therefore aimed at optimizing conditions for the preparation of AC from RH for humic acid removal from water. This involved, i) characterization of the RH varieties in Uganda, ii) optimization of pyrolysis conditions, iii) optimization of physical and chemical activation conditions, and iv) determination of the adsorption performance of the prepared activated carbons towards humic acid removal from water. Upland rice husk varieties were found more suited AC precursors, due to their lower ash content, higher volatile matter and fixed carbon contents. For purposes of sufficiently lowering the ash content of the RH to less/or equal to 5% dry basis (db), a NaOH-concentration of 2-4%w/v was found appropriate for use in the NaOH-pretreatment process. Optimum pyrolysis conditions were found to be temperature (406 oC), heating rate (10 oCmin-1), and heating period (60 min), resulting in char yield, fixed carbon and ash contents of 35.3, 55.4, and 35.0%db, respectively. Based on the product value of carbon yield, C, and the total specific surface area, as,BET, the best physically activated carbon resulted from char activation at a temperature of 800 oC, for 10 min. At these conditions, the C, as,BET, and the C×as,BET values were obtained as 53.2%, 756.8 and 402.7 m2g-1, respectively. The best chemically activated carbon (AC 400) resulted from rice husk activation at a H3PO4 concentration of 30wt%, and at a temperature of 400 oC. At these conditions, the C, as,BET,, and the C×as,BET values were obtained as 46.9%, 2258.4 and 1058.7 m2g-1, respectively. Despite the higher as,BET value, its maximum adsorption capacity (5.3 mgg-1) was lower than 8.9 and 27.2 mgg-1 exhibited by the ACs prepared at a H3PO4 concentration of 30 wt% and at activation temperatures of 500 (AC 500) and 600 oC (AC 600), respectively. Therefore, other factors rather than surface area alone had significant influence over humic acid removal. The best conditions for humic acid removal included; adsorbent dose (0.5 g), contact time (70 min), and solution pH (2). The adsorption capacity of AC 600 was comparable to that of the commercial AC. Overall, the study revealed that the RH in Uganda have good prospects for utilization in AC production for humic acid removal from water.