Production and optimisation of adsorbent materials from teawaste for heavy metal removal from aqueous solution: feasibility of hexavalent chromium removal – kinetics, thermodynamics and isotherm study

A face-centred central composite experimental design scheme was used to find experimental conditions which would result in the Magnetic Bleached Teawaste (MBTW) adsorbent material produced have the optimum hexavalent chromium (Cr(VI)) removal capacity. Effect of treatment conditions, namely temperature (200 °C, 350 °C, 500 °C), duration of the heat treatment (2 h, 3 h, 4 h), the concentration of the iron chloride (0.02 g/g, 0.055 g/g, 0.09 g/g) added was investigated. The results obtained showed that the pyrolysis temperature and the amount of iron chloride added during treatment had significant effects on the performance of MBTW material in removal of both total chromium and Cr(VI). Increasing temperature from 200 °C to 500 °C, both Cr(VI) and total Cr removal would decrease. The material with optimum removal of Cr(VI) was used for the detailed adsorption study. The results from the detailed adsorption study indicated that the MBTW material has an excellent Langmuir removal capacity of Cr(VI) (269.9 mg/g). The Cr(VI) removal process also involved the reduction of the Cr(VI) to Cr(III). The removal process was shown to be temperature-dependent and endothermic. According to the FTIR analysis and the kinetic modelling results, chemisorption is the main mechanism responsible for the removal of the Cr(VI). The effect of background ions on the reduction of Cr(VI) was also investigated and the results show that presence of carbonate ions caused a huge reduction in removal of Cr(VI) whilst chloride ions had the least effect.