Radiation grafted nanofibrous adsorbent containing N-methyl-D-glucamine for boron removal

Abstract

Ion exchange resins having glucamine groups, although bearing a great potential in treating varieties of boron-containing streams have slow kinetics due to mass transfer limitations. The objective of this study is to develop a new adsorbent with a fibrous morphology that gives high adsorption capacity and fast kinetics. The new adsorbent was prepared in 3 steps involving electrospinning of poly(vinylidene difluoride) (PVDF) into nanofibres, radiation induced grafting (RIG) of glycidyl methacrylate (GMA) onto electrospun nanofibres followed by functionalisation with N-methyl-D-glucamine (NMDG). Response surface methodology (RSM) was used for optimization of PVDF electrospinning parameters allowing fibres’ diameters control. Simultaneous RIG was performed with an electron beam under controlled parameters. The functionalisation reaction’s parameters were also tuned with RSM to maximize the NMDG density in the adsorbent. The nanofibrous adsorbent was characterized using scanning electron microscopy, Fourier transform infrared spectrometer, differential scanning calorimetry, thermogravimetric analysis and water contact angle measurements. The performance of the adsorbent was tested for boron removal under batch and dynamic column (fixed bed) modes. The stability of the new adsorbent was confirmed by sorption/desorption tests. Nanofibrous sheets with an average fibre diameter of 350 nm were obtained at optimum voltage and concentration of 15.5 kV and 15 wt%, respectively. An optimum degree of grafting (DG) of 150% was imparted in grafted PVDF nanofibres using a 90% GMA/methanol solution and a dose of 40 kGy at a dose rate of 1.27 kGy/s. A maximum NMDG density of 2.20 mmol/g was achieved at optimum parameters of 15% NMDG concentration, 86.9 °C reaction temperature, 64.7 min and 150% DG. The new adsorbent showed 100% removal efficiency using a 0.6 g adsorbent dose within 2 h for a 100 mg/L of boron solution. The adsorption data from batch mode were best fitted to the Redlich–Peterson isotherm and the adsorption kinetics followed the pseudo-second-order. The adsorbent behaviour under dynamic conditions revealed that the breakthrough capacity is a function of both initial feed concentration and bed height whereas the flow rate marginally affected the breakthrough capacity as indicated by the 9.3% reduction with an increase of up to SV 200h-1. The Thomas mathematical model was found to best fit the dynamic behaviour of the column. The adsorbent displayed a boron adsorption capacity of 17.60 mg/g-adsorbent which is 2.6 time higher than that of commercial boron selective resin such as Amberlite IRA743 (6.7 mg/g). The results of this study suggest that the adopted preparation procedure is highly effective in preparation of nanofibrous adsorbents with the desired content of boron selective ligands. Moreover, the adsorbent was proven to have a strong potential for application in boron removal from solutions as indicated by higher boron adsorption capacity and faster kinetics.