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A novel mercapto-grafted rice straw (RS-GM) was synthesized and characterized against raw rice straw (RS-Raw). The RS-GM yielded a better Hg(II) biosorption performance compared to RS-Raw as the incorporated-mercaptan functional group presented a strong affinity toward Hg(II). The isotherm data correlated well with the Langmuir isotherm model having maximum biosorption capacity, q max of 103.10 mg/g and 161.30 mg/g for RS-Raw and RS-GM, respectively. The Hg(II) biosorption is thermodynamically feasible, endothermic, and spontaneous in nature. The biosorption kinetic data were best-fitted into the pseudo-second-order kinetic model. Further analysis suggested that the Hg(II) biosorption process is governed by the intraparticle and external mass transfer in which the film diffusion was the rate limiting step. The adsorption–desorption cycle was repeated four times by using diluted HCI as desorbing agent, and the results were comparable to the unused biosorbent. The removal of Hg(II), which was also done in the produced water, was found to be 96% by using RS-GM.

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Desiccated coconut waste sorbent (DCWS), a byproduct of coconut milk processing, was studied as a sorbent for Hg(II) sorption. Energy dispersive X-ray analysis indicated that the DCWS mainly contained C, N, and O, while the Fourier transform infrared measurements confirmed the existence of hydroxyl, carboxyl, and amine groups on the DCWS surface. The point of zero charge (pH pzc ) and cation-exchange capacity (CEC) values were 6.05 and 2.02 meq/100g, respectively. The batch equilibrium data were fitted well by the Langmuir isotherm model with a maximum sorption capacity, q max of 500 mg/g, while the kinetic sorption data were found to follow a pseudo-second-order kinetic model. A column sorption study showed that the sorption capacity increased and the breakthrough time decreased with the increase in the initial Hg(II) concentrations. The regeneration studies revealed that the DCWS could be regenerated and reused.

This paper presents the study of the adsorption characteristics of sulfur-functionalized silica microspheres (S-SMs), synthesized through co-condensation of tetraethyl orthosilicate with 3-mercaptopropyl trimethoxysilane (MPTMS) and bis(triethoxysilylpropyl) tetrasulfide (BTESPT) as sulfur ligands, with respect to the removal of Hg(II) from aqueous solutions. The synthesized adsorbents were characterized using a scanning electron microscope, an X-ray diffractometer, a nitrogen adsorption−desorption analyzer, a Fourier transform infrared spectrophotometer, and an energy dispersive X-ray diffractometer. The effects of pH, concentration, temperature, stirring time, and adsorbent reusability were studied via batch adsorption experiments. It was found that the optimal adsorption pH values for all synthesized adsorbents were between 5.8 and 8.2. The adsorption capacity of SMs was 20.0 mg/g and increased to 37.0 and 62.3 mg/g for BTESPT-SMs and MPTMS-SMs, respectively. Hg(II) adsorption was found to be exothermic in nature and followed the chemisorption mechanism. The Langmuir isotherm model was found to be the best fitted model for describing the isotherm data, while the kinetic data obeyed the pseudosecond-order kinetic model, in which film diffusion was found to be the rate-controlling step. The regeneration study using potassium iodide as a regeneration agent showed high reusability, up to five-cycle activity.

Polyvinyl alcohol (PVA) hydrogel are still restricted for some applications because their lower mechanical strength and thermal stability. The PVA-based composites are drawing attention for the removal of heavy metals based on their specific functionality in adsorption process. The main objective of this work is to synthesize oil palm bio-waste (OPB)/multiwalled carbon nanotubes (MWCNTs) reinforced PVA hydrogels in the presence of N,N′-methylenebisacrylamide (NMBA) as a crosslinking agent and ammonium persulfate (APS) as an initiator via simple in-situ polymerization technique. The as-prepared reinforced nanocomposites were characterized by FESEM, BET surface area, differential scanning calorimetry (DSC), TGA and FTIR analysis. The possible influence of OPB and MWCNTs on the tensile strength, elongation at break and elastic modulus of the samples were investigated. It was found that reinforced nanocomposites exhibited enhanced mechanical properties as compared to non-reinforced material. The evaluation of reinforced nanocomposites was tested by the removal of Pb(II) aqueous solutions in a batch adsorption system. The pseudo-second-order kinetic model was used to illustrate the adsorption kinetic results and Langmuir isotherm was more suitable to fit the equilibrium results providing maximum adsorption capacities. The evaluation of thermodynamic parameters describes the spontaneous, endothermic and chemisorption adsorption process while activation energy reveals the physical adsorption mechanism. Therefore, the coordination effects among OPB, MWCNTs and PVA polymer hydrogels can produce a promising adsorbent material for wastewater treatment applications.

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