Plasma modified steel processing by-product for removing heavy metals and antibiotics from water

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The presence of heavy metals (HMs) and antibiotics (ABs) in the aquatic environment causes critical problems to human health and the environment. The adsorptive removal of HMs and ABs onto cost-effective adsorbents has a high potential. In this study, adsorbents were prepared from steel shavings (StS), a by-product generated from the steel processing industries. Among adsorbents, nitrogen plasma modified StS (M₃-plN₂) has highest adsorption capacities of HMs and ABs. Adsorption and co-precipitation were the mechanisms for HMs removal by the adsorbents, while main driving forces for ABs adsorption were hydrogen bonding, electrostatic and non-electrostatic interactions, and redox reaction. Thermodynamic data demonstrated that both adsorption processes of HMs and ABs onto the adsorbents were feasible, spontaneous and endothermic. Solution pH, particle size, adsorbent dose and contact time exerted great influences on the adsorption process. Optimal conditions for the adsorptive removal of HMs were pH 5, adsorbent dose 5g/L, at 25℃. The best removal of sulfamethazine (SMT) and chloramphenicol (CP) was observed at pH 3, while tetracycline (TC) was ultimately removed at pH 5 (with the same adsorbent dose of 2 g/L and at 25℃). The Pseudo-first-order kinetic and Pseudo-second-order kinetic models described the adsorptive kinetics of HMs and ABs very well. The Langmuir maximum single adsorption capacities of Pb²⁺, Cu²⁺, Cd²⁺, Cr³⁺ and Zn²⁺ onto M₃-plN₂ were: 27.04, 20.64, 16.87, 14.89 and 18.47 mg/g, respectively. In competitive adsorption of multi-metals solutions, each competitive solute adsorption capacities were approximately 2-fold less than the single adsorption capacities. However, the total of competitive adsorption capacities was higher than those of single solute sorption. Single Langmuir adsorption capacities of SMT, TC and CP onto M₃-plN₂ were 2702.55, 2158.36 and 2920.11 μg/g, respectively. Adsorption capacities of mixed-ABs onto the adsorbents were nearly 2-fold less than individual adsorption capacities. Furthermore, the metals-loaded M₃-plN₂ was well regenerated using sulphuric acid 0.1N after 5 cycles of adsorption-desorption, while the most effective reagent to regenerate ABs-loaded M₃-plN₂ was methanol 0.1N solution after 2-3 adsorption-desorption cycles. The semi-pilot scale experiments confirmed that fixed-bed column using M₃-plN₂ could efficient abate both HMs and ABs from water with the highest removal efficiencies at a flow rate of 3.47 L/min and bed height of 35 cm. The column adsorption data was well described by The Thomas, Yoon-Nelson and BDST models. Overall, the application of M₃-plN₂ for removing HMs and ABs from aqueous solution can provide tremendous benefits in treating water and reducing solid wastes.
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