| In this study, carboxyl groups were introduced onto the surface of maize straw to obtain the hydrogen butanedioate modified sorbents for chelating with divalent metals. Subsequently, the magnetic derivative was obtained. The synthetic conditions, adsorption, separation, and recovery of heavy metals were studied in details. It provided a new idea to prepare maize straw-based functional materials, leading to high value-added maize straw-based materials with potential applications. It was also expected to solve the problem of the discarded maize straw, which was incinerated or abandoned to lead to resource waste and air pollution.Maize straw was modified with succinic anhydride for the introduction of carboxyl functional groups. The succinylated-maize straw (S-MS) was characterized by FTIR and solid-state MAS 13C NMR spectroscopy. The product with the concentration of carboxylic fucntions (ncooH) value of 5.8 mmol/g was obtained under the optimal synthesis conditions.The magnetic succinylated-maize straw (M-S-MS) was obtained from succinylated-maize straw and amino-functionalized Fe3O4 (NH2-Fe3O4) with N,N’-diisopropylcarbodiimide (DIC) as catalyst. Under the optimal synsthesis conditions, the concentration of carboxylic fucntions (ncooH) and amino values of the product were 4.2 mmol/g and 7.4 ×10-2 mmol/g, respectively. The product was characterized by IR. The saturated magnetization of M-S-MS reached 21.3 emu/g, and as a result, it could be separated from aqueous solution only for 10 s by a magnetic process for its superparamagnetism.After deprotonating the carboxylic acid groups of S-MS and M-S-MS, respectively, two adosorbents based on maize straw were obtained. Batch experiments were carried out with the two adsorbents for the removal of Cd(II). According to XPS and FTIR analysis, the adsorption process of Cd(II) on the two adsorbents was ion-exchange and chemisorption with complexation. The influence of various experimental parameters were investigated. After mixing with aqueous solutions for 1 min, the adsorbent, NaS-MS, showed the adsorption capacity of 95.5 mg/g-200.1 mg/g, while the amount of Cd(II) adsorbed was 54.5 mg/g-71.1 mg/g for M-NaS-MS. At 298 K, the experimental data were best described by Langmuir adsorption model and the maximum adsorption capacity of the two adsorbents were 196.1 mg/g and 85.5 mg/g, respectively. The chemical nature of the adsorption can be determined by calculating the mean free energy of adsorption of ’E’from D-R model and the high correlation coefficients (R2) for the pseudo-second-order kinetic model. Thermodynamic parameters were calculated from data obtained from experiments performed to study the effect of temperature.Both adsorbents could be regenerated by 5 mol/L NaCl solution. After regenerated five times in saturated NaCl solution, the adsoption capacity of the two adosrbents did not decrease and the saturated magnetization of M-NaS-MS did not reduce. Furthermore,～97% of the adsorbed Cd(II) by NaS-MS could be recovered as its metal oxide.As a continuation of the studies, the adsorbent, NaS-MS, was used to investigate the competitive adsorption behavior of Cd(Ⅱ) and Ni(Ⅱ) in a binary system. The effects of various factors, such as contact time, pH, multi-metal isotherms, and adsorbent dose on the simultaneous removal of the two metal ions were studied. The separation factor of Cd(Ⅱ)/Ni(Ⅱ) reached as high as 8.6 after 1.5 h of contact time at pH 4-6 with an adsorbent dose of 0.6 g/L when the initial concentration of both Cd(Ⅱ) and Ni(Ⅱ) were 1 mmol/L at 298 K. The CdO and NiO mixture was obtained after the biosorbent was stirred with the binary mixture and calcined in a muffle furnace. The reaction activity of the mixture differed at varying HCl concentrations. Competitive adsorption could be applied to acid-leaching solutions. Therefore, a potential separation procedure was also proposed to recover pure Cd(Ⅱ) and Ni(Ⅱ) from a binary mixture with various molar ratios of Ni(Ⅱ) to Cd(Ⅱ). This technique was then successfully employed to separate Cd(Ⅱ) and Ni(Ⅱ) ions from the simulated leach liquor of spent Ni-Cd batteries. Approximately 90% of the adsorbed Cd(Ⅱ) ions were recovered, and the resulting solution contained >99 wt% of Ni(Ⅱ). |
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