Study On Kinetics And Mechanism Of Biosorption Of Cadmium From Simulated Wastewater By Tea Waste

In the recent twenty years, biosorption and removal of contaminants from wastewater using waste biomass is a cost-effective and environmentally friendly water treatment technology. Cadmium (Cd) is one of the greatest threats to human health in all toxic trace elements. Batch experiments were carried out to evaluate the effect of some factors such as pH, contact time, biosorbent dose, particle size, ionic strength and temperature on the biosorption of Cd onto tea waste. In order to explore the adsorption mechanism, the impact of surface functional groups on tea waste to the adsorption process was discussed. A better modified process was determined by investigating the effect of various modified methods on the biosorption of Cd from simulated wastewater by tea waste. The adsorption mechanism of modified tea waste biosorption of Cd was explored. The main research results were shown as follows:1) The adsorption kinetics was satisfactorily described by pseudo-first order and pseudo-second order model. The experimental equilibrium data could be well fitted to Langmuir and Freundlich isotherms. The maximum biosorption capacity was found at pH7. Biosorption of Cd increased with the decreasing size fraction and dosage of tea waste. The adsorption of Cd on to tea waste was less inhibited by Na+, K+than by Mg2+, Ca2+and Pb2+. Thermodynamics studies revealed that biosorption of Cd onto tea waste was spontaneous (△G°<0) and endothermic (△H°>0) process. Adsorption process was dominated by chemical adsorption. The dynamic adsorption data of biosorption of Cd could be well fitted to Thomas isotherm and q0close to the true value.2) The initial adsorption rate of Cd can beimproved by NaCl modification, but the amount of adsorption can not be increased; Ethanol and ethyl acetate modified tea waste had no significant effects on the initial adsorption rate and adsorption capacity of Cd compared with the original tea waste. HNO3and citric acid modified tea waste had no significant difference on initial adsorption rate, but adsorption capacity was less than original tea waste. NaOH, KOH and Ca (OH)2modification process can significantly improve the amount of adsorption. The maximum adsorption capacity of Cd increased with the increasing of concentration of KOH in modified process. Therefore,0.1mol/L KOH modified process might be a better way of modification.3) Tea waste after KOH modification could improve the range of optimum pH. Thermodynamics studies revealed that biosorption of Cd onto KOH modified tea waste was spontaneous (△G°<0) and endothermic (△H°>0) process. The dynamic adsorption data of biosorption of Cd onto KOH modified tea waste could be well fitted to Thomas isotherm, and adsorption capacity of KOH modified tea waste was three times more than that of original tea waste. Complexing capacity of surface groups on KOH modified tea waste was between EDTA and citric acid. KOH modified tea waste might be used not only to the biosorption of Cd from simulated wastewater, but also be used to biosorption of other heavy metals.4) Dissolution of K was unrelated to adsorption capacity of KOH modified tea waste in different initial concentrations of Cd. Compared with FTIR spectra between biosorption of Cd onto original tea waste and KOH modified tea waste, phenolic hydroxyl and carboxyl groups were predominant contributors in biosorption. There was significant positive correlation between the amount of carboxyl group after acidification and adsorption capacity. Among the relationship of total negative groups of KOH modified tea waste, total acidic groups of KOH modified tea waste after biosorption of Cd and adsorption capacity of KOH modified tea waste, it implyed that hydroxyl and carboxyl groups were predominant contributors in biosorption.