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Cationization of agricultural by-products using metal salts is widely used to activate their phosphorous capture ability. This study developed three kinds of new metal loaded soybean milk by-product 'okara' for phosphorus biosorption. A comparative study among these biosorbents was carried out with respect to their performances in terms of affinity, stability and reusability. Zirconium loaded okara (ZLO) was found to have the highest affinity towards PO(4)(3-) anions (47.88 mg/g), followed by iron/zirconium loaded okara--IZLO (40.96 mg/g) and iron loaded okara--ILO (16.39 mg/g). ZLO was successfully desorbed with 0.2M NaOH and activated with 0.1 HCl prior to the next cycle. After five consecutive cycles, the efficiency of both adsorption and desorption of ZLO remained about 85% whilst no Zr(IV) leakage was observed. Conversely, IZLO and ILO suffered from vital short comings such as high metal release and/or sharp reduction in PO4(3-) sequestering capability after multi operation cycles.

This critical review discusses the potential use of agricultural waste based biosorbents (AWBs) for sequestering heavy metals in terms of their adsorption capacities, binding mechanisms, operating factors and pretreatment methods. The literature survey indicates that AWBs have shown equal or even greater adsorption capacities compared to conventional adsorbents. Thanks to modern molecular biotechnologies, the roles of functional groups in biosorption process are better understood. Of process factors, pH appears to be the most influential. In most cases, chemical pretreatments bring about an obvious improvement in metal uptake capacity. However, there are still several gaps, which require further investigation, such as (i) searching for novel, multi-function AWBs, (ii) developing cost-effective modification methods and (iii) assessing AWBs under multi-metal and real wastewater systems. Once these challenges are settled, the replacement of traditional adsorbents by AWBs in decontaminating heavy metals from wastewater can be expected in the future.

Palm oil fruit shells were evaluated as a new bioadsorbent to eliminate toxic copper from water and wastewater. Without any chemical treatment, palm oil fruit shells were washed, dried and grounded into powder (75 μm) for use in the experiments. Characterization showed mesopore based bioadsorbent was prepared from palm oil fruit shells. The results indicate that the highest Cu removal efficiency was found in an aqueous solution with pH of 6.5. The equilibrium sorption capacity of copper was significantly high (between 28 and 60 mg/g) at room temperature. Nonlinear regression analyses for isotherm models revealed that three-parameter isotherms had a better fit to the experimental data (R(2)0.994) than that of two-parameter isotherms. The copper sorption system was heterogeneous as the values of exponents were lying between 0 and 1. The highly correlated pseudo-second-order kinetics model (R(2)0.998) ascertained the applicability of copper removal by palm oil fruit shells.