Synthesis and characterization of novel green chemicals from rice husks and their technological applications in environmental pollution control in Kenya

Abstract

Rice husks (RH) are products of rice milling. Waste management of RH creates health and environmental problems. Wastewater discharges lead to serious environmental problems. The goal of the study was to synthesize and characterize industrial chemicals using RH, test their technological application in wastewater treatment especially lead, chromium and cadmium removal in water. RH was collected from rice millers in Kirinyaga County, whereas wastewater samples were collected from tea factory in Kiambu County, Kenya, A oxidation reactor for converting RH to RHA was developed. Optimum conditions for sodium silicate (SS) and silica gel (SG) production were determined by separately leaching with 0.5 M HCl and 0.5 M H₂SO₄. Silica (S) obtained was reacted with 1 M, 2 M, 3 M NaOH to find optimum conditions for SS production. SS solutions were further reacted with 3 M HCl each to determine ideal conditions for silica gel (SG) production. To prepare RHACs, RH was carbonized and residual char activated with sodium hydroxide (NaOH) and phosphoric acid (H3PO4). Resulting SS, SG, and RHACs samples were analyzed and characterized using TXRF, SEM, XRD, and infrared spectroscopy. Heavy metal removal efficiency of RHACs was tested by mixing aliquots of wastewater; adsorbent (RHAC), centrifuged at 4500 rpm then using UV spectrophotometry (765 nm). Removal efficiency and kinetics of removal of Pb, Cd, and Cr using RHACs evaluated using AA spectroscopy. RH had a moisture content of 7.07 ± 0.63 %. Percent ash content ranged from 23.43 ± 0.77 to 22.17 ± 0.48. Leaching (0.5 M HCl ) RH and thermal treatment at 600 °C yielded the best % SiO2 and removed the most metallic impurities (P2O5, K2O, CaO, MnO, ZnO). To determine the optimum conditions for the conversion of RHA to SS and SG, RH was converted to RHA at different temperatures (400, 500, 600 °C), 60 g leached RHA was then reacted with 300 mL of 1 M, 2 M, 3 M NaOH. Infrared spectroscopy analysis indicated HCl leaching availed more silicon dioxide. Post leaching RHA yielded 97.37 SiO2 % (water), 99.17 SiO2 % (HCl), and 99.02 SiO2 % (H2SO4) silicon dioxide respectively. SS (Liquid neutral) from RH analyzed at KEBs compared well with KS2350:2012 Standard. Results show NaOH-RHACs were more efficient in total phenols removal. The total phenolic removal efficiency from factory tea wastewater was 72-77 % for NaOH-RHACs compared to 2.6% for H3PO4-RHACs. Langmuir correlation coefficient R2 (0.99) for Pb (II), Cd (II) and Cr (ll) removal using NaOH and H3P04-RHACs indicated monolayer adsorption. Kinetic pseudo-second-order R2 (0.90) values indicated electron transfer between solution and adsorbent. Processing of RH wastes into innovative novel chemicals that have industrial applications is a sustainable approach of managing the wastes. Silica, silica gel and RHACs can be manufactured from RH due its high silica content. Such chemicals find wide spread applications in wastewater management. This development of wastewater treatment chemicals can mitigate the problems caused by heavy metal pollutants.