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.

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The persistence of antibiotics in sewage wastewater treatment plants in recent years has emerged as a serious issue. This study therefore evaluated three types of natural zeolites (i.e., NZ01, NZ02, and NZ03) for the removal of selected antibiotics namely tetracycline (TC) and oxytetracycline (OTC) from aqueous solution. The physical and chemical properties of the natural zeolites were characterized using various analytical techniques and the antibiotic removal studies were carried out in a batch adsorption experiment evaluated at various experimental conditions. The highest adsorption capacity was 62.5 and 76.3 mmol/g for TC and OTC, respectively, observed for NZ02 at pH values between 7 and 8. The adsorption of TC and OTC followed the Langmuir and Temkin isotherm models, respectively, whereas the adsorption kinetic data obeyed the Elovich kinetic model. The regeneration study on NZ02 indicated that it can still remove TC and OTC after five adsorption/desorption cycles. The removal efficiency of TC and OTC also improved from approximately 5-39 and 28%, respectively, by addition of NZ02 during the coagulation/flocculation process. These results thus demonstrated the applicability of the natural zeolites, especially NZ02, for the removal of TC and OTC from aqueous solution; hence the potential for the application in sewage treatment process for removal of antibiotics through either conventional adsorption/desorption or adsorptive coagulation/flocculation (ACF) process.

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