NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- Due to the rapid industrialization, heavy metals have been continuously and excessively released into the environment, generating remarkable impacts on human and other organisms. Heavy metals such as cadmium, zinc, copper, nickel, lead, mercury and chromium are often detected in industrial wastewaters, causing an obvious or potential threat to water quality, human health and biodiversity in the ecosystems. In consideration of the toxicity of heavy metals to life forms, heavy metals have been prioritised as major inorganic contaminants in the environment. Due to this increased awareness and concern about environmental pollution, stringent national and international legislations has been set up, generating more efforts of research work in this area, especially in finding better and more efficient technique to treat wastewater bearing heavy metals. The search for new technologies involving the removal of toxic metals from wastewaters has directly focused on biosorption, basing on metal binding capacities of various materials (e.g. algae, fungi, yeast, and bacteria) which are available in large quantities, or certain industrial or agricultural by-products, that can become suitable biosorbents after modification. The objective of this study was to develop novel low-cost biosorbents for removing heavy metals from water and wastewater.
Among these various materials, agricultural by-products have been studied most extensively. For one single year, agricultural by-products as a whole exceed 320,000,000 tonnes, offering inexhaustible materials and great selectivity for biosorption investigation. Besides, agricultural by-products possess various advantages, such as non-hazardous, relatively cheap, high biosorption potential, specifically selectivity and easily disposed by incineration. Therefore, in this study, three common agricultural by-products, banana peel, sugarcane bagasse and watermelon rind have been used for biosorption of Copper (Cu), Zinc (Zn) and Lecd (Pb) from water as novel biosorbents. The dried and ground biosorbents have been contacted with metal solutions so as to investigate optimal pre-treatment conditions (e.g. drying temperature, drying time and particle size), impact of major influencing factors (e.g. biosorbent dosage, initial metal concentration, pH, co-existence ions) and desorption processes. Equilibrium studies and kinetic modelling were investigated for the determination of potential in industrial-scale applications. BET and SEM characterization of these three biosorbents were also conducted so as to study the major mechanisms in the biosorption process.
The optimal conditions were determined in terms of Cu removal efficiency and/or energy consumption. The results indicated that the banana peel dried at 120°C for 2 h and grounded into powder form led to a better performance in terms of both copper removal efficiency and energy consumption. For both sugarcane bagasse and watermelon rind, 120°C was the suitable drying temperature. However, the best drying time was 1 h for sugarcane bagasse and 3 h for watermelon rind. The power form with the size of less thanl50 pm was optimal for all three biosorbents in terms of removal efficiency and equilibration time. 0.5g was found to be suitable biosorbent dosage for batch experiments while metal concentration with 10mg/L was also determined for the further experiments.
pH had been found to affect biosorption considerably in the range of 2 to 6.8. The optimum sorption capacity of Cu, Zn and Pb were found to be 6, 6.8, 6.8, respectively, when using banana peel as a potential biosorbent. In case of watermelon rind, the maximal removal efficiency of Cu, Zn and Pb occurred when pH value was 5, 6.8, 6.8, respectively. For sugarcane bagasse, pH 6.8 was suitable for all three metals. From the results of co-existence- ion experiments conducted, uptake of all three metals decreased with Zn being inhibited to the largest extent.
In the desorption study, distilled water was found can either desorb metal ions or increase the biosorptive of the biosorbents. Acid eluants showed significant advantages in the metal recovery. Almost 100% of metals ions can be recovered. However, after acid desorption, the biosorptive capacity reduced significantly. While for alkaline eluant, it not only recovered most of the sorbed ions, but also increased the biosorptive capacity. After the first cycle, the removal efficiency reached as high as 99% and remained constant for all consecutive cycles. Desorption and re-sorption studies of Cu, Zn and Pb in multi-metal solution were also carried out and similar results were found. After desorption in NaOH solution, the re-sorption efficiency reached as high as 99% of these three metals either in a single-component or multicomponent system. Besides, no significant competition between Cu, Zn and Pb ions was found during desorption process. Based on the results, NaOH showed significant advantage compared to other eluants and was considered as the most suitable eluant in this study.
The higher proportion of the heavy metal ion was sorbed during 1-10 min of contacting and equilibrium had been reached within 60 minutes. Two kinetic models (e.g. pseudo first-order kinetic model and pseudo second-order kinetic model) were applied to the biosorption process and high correlation coefficient favoured pseudo second-order kinetic model. Langmuir, Freundlich and Redlich-Peterson adsorption models were used in the equilibrium study to fit the equilibrium data procured after 10 hours of contacting. As to the Langmuir isotherm model, maximum biosorption capacity of banana peel for Cu, Zn and Pb was found to be 10.10, 6.17, 142.85 mg/g, respectively. For sugarcane bagasse, biosorptive capacity of Cu, Zn and Pb was 10.64, 4.05, 122.75 mg/g, respectively whilst for watermelon rind, biosorptive capacity of Cu, Zn and Pb was 6.28, 6.85, 92.88 mg/g, respectively. Base on higher values of r2, Redlich-Peterson model was the best fit for the obtained data. For better understanding and estimation, non-linear Chi-square analysis method was used for determination non-linear models. Compared with linear models, the obtained results indicated that non-linear model provided a better fitness for the three sets of experimental data.
Based on the SEM and BET characterization results of these three biosorbents, ion-exchange and micro-precipitation occurred on the surfaces of biosorbents were believed to be the main mechanisms.
Thus, all these obtained results indicated that banana peel, sugarcane bagasse and watermelon rind possessed great potential to become excellent biosorbents for removing heavy metals from water and wastewater.