GREEN TECHNOLOGY FOR WASTEWATER MANAGEMENT: UTILIZING RICE HUSKS, COCONUT SHELLS AND CLAY AS ADSORBENTS FOR BASIC DYE RHODAMINE B AND SELECTED HEAVY METALS

Abstract

Accumulation of heavy metals such as cobalt, copper, and lead and dyes such as rhodamine B (RB) released by various industries, poses a potential ecological risk of contamination due to their persistent nature and non-biodegradability. A remedial action is needed to mitigate this risk. A study was carried out to look into the possibility of eco-friendly and cost effective activated rice husk charcoal (ARC), coconut shell charcoal (ACC) and activated clay soil (ACS) in removal of heavy metals and rhodamine B dye from waste water. Three metal ions were selected, Cu2+, Co2+

and Pb2+ and were determined using atomic absorption spectrophotometer, AAS. The charcoal was obtained by burning rice husk and coconut shells which had been soaked in phosphoric acid in limited air. The ability of adsorbents to adsorb metal ions from water was investigated by carrying out batch experiment and varying parameters such as initial concentration, adsorbent dose, contact time, agitation speed, temperature and pH. Fourier Transform Infrared (FTIR) analysis detected presence of functional groups of Si-OH, Si-O, Si-O-Si, O-H and C=C in adsorbents upon activation which are responsible for adsorption. For the dye, a characteristic peak at 3200 cm-1

to 3070 cm-1

for aromatic stretch which was confirmed by a stretching vibration peak at around 1400 cm-1 was observed after adsorption. X-ray diffraction (XRD) data indicated that ACS was crystalline while ARC and ACC were amorphous. The X-ray fluorescence (XRF) results show that SiO2, Al2O3, and K2O were the main components where SiO2 were dominant especially in ARC and ACS. With Transmission emission spectroscopy (TEM) analysis, these nanostructures showed significantly different morphologies. Highly porous and hollow structures were visible in all cases though with differences. Brunauer, Emmett and Teller (BET) surface area and average pore diameter determination study showed that there was a relatively equal adsorption-desorption efficiency. This indicated that the adsorbents were highly reusable and that they had good stability. The efficiency of all the adsorbents towards the removal of heavy metals and dye increased with increase in temperature, time and surface area. Varying the pH showed that metals were adsorbed at a maximum pH of 6 while RB was highly adsorbed at pH 4. The equilibrium fitted the Langmuir equation well as compared to Freundrich. The R2

values for Langmuir ranged between 0.9928 to 0.9989 as compared to Freundlich which ranged between 0.9200 to 0.9671. The negative values of change in Gibb’s free energy (-1.55 to -0.581, -1.074 to -0.449 and -1.14 to -0.194 kJ mol-1

for ARC, ACC and ACS respectively) and positive values of change in enthalpy (32.034 to 37.562, 30.615 to 36.193 and 30.08 to 34.76 kJ mol-1

for ARC, ACC and ACS respectively) revealed the process to be spontaneous and endothermic. The use of a pseudo-second-order model, which indicates that chemical adsorption governed the process, allowed for a more accurate description of the adsorption process.