New Selective Adsorbent from Modified Waste Nylon-6 Microfibers for Removal of Boron from Waters
Authors: Ting Teo Ming , Nasef Mohamed Mahmoud 
Published: July 2019

The wide spread application of boron in various industries such as glass and fiberglass, ceramics, abrasives, detergents and soaps, fertilizer, enamels, insecticides, semiconductors, cosmetics and pharmaceuticals has left many surface and waste water streams polluted. Thus, there is a growing global demand for new chelating materials and efficient separation systems for removal of boron from different water streams. This is because the existing boron removal technologies are challenged by slow performance coupled with high treatment cost caused by strict low boron concentration required in water bodies and discharged wastewater to meet the newly imposed regulation [1]. Selective adsorbents obtained by modification of polymeric fibres with radiation induced grafting of functionalized monomers are potential materials for improving the performance of current ion exchange systems operated based on granular chelating resins for boron removal to desired low levels [2]. In this work, a new adsorbent having microfibrous structure was prepared by radiation induced grafting of 4-chloromethylstyrene (CMS) onto nylon-6 fibres waste followed by functionalisation with N-methyl-D-glucamine (NMDG) and testing for boron removal from solutions in batch and continuous column modes as schematised in Fig. 1. The degree of grafting (DOG) in the adsorbent precursor was tuned by variation of reaction parameters and and optimum DOG of 130% was achieved at a CMS concentration of 20 vol% in methanol, a total dose of 300 kGy, a temperature of 30 °C and a reaction time of 3 h. A maximum glucamine density of 1.7 mmol/g was loaded in the adsorbent at 121% DG, 10.60% NMDG concentration, 81 °C reaction temperature and 47 min reaction time. The chemical composition, morphology and structural changes in nylon-6 fibres caused by grafting of CMS and subsequent glucamine treatment were monitored by Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The thermal properties were determined using differential scanning calorimetry (DSC) and the thermal stability was evaluated by thermogravimetric analysis (TGA). The mechanical properties were investigated with a universal mechanical tester. The obtained fibrous adsorbent displayed an increase in the average fibre diameter compared to original and grafted fibres (Fig. 2). The adsorbent showed 100% removal efficiency for boron removal from solutions at an initial concentration of 100 mg/L, temperature 30 °C, reaction time 2 h and pH of 7. The new adsorbent can also achieve a maximum adsorption capacity of 13.8 mg/g at pH 7, which is 20% higher than that of commercial granular resins. The adsorption isotherm of boron on the fibrous adsorbent was best fitted to Redlich-Peterson isotherm model whereas the adsorption kinetic behaviour is well fitted by the pseudo-second-order model. The new fibrous adsorbent also showed rapid kinetics compared to commercial resin as indicated by the reduction in the adsorption equilibrium time from 60 min for commercial resins to 30 min. The breakthrough curves obtained from column studies conducted under dynamic conditions (initial concentration 10 mg/L and pH 7) shown in Fig. 3 suggest that the fibrous adsorbent is about 2.2 times faster than granular resin. The adsorption capacity of boron remained almost constant after five cycles of adsorption/desorption cycles suggesting a good chemical stability. Considering the essential properties such as high external surface area, rapid kinetics, high adsorption capacity, mechanical strength and chemical stability, it can be suggested that the new fibrous adsorbent obtained from nylon-6 fibres waste is highly promising for boron removal from solutions. The technology developed in this work can be harnessed for preparation of various types of adsorbents for chemical decontamination of industrial waste water, surface water and underground water using a variety of waste polymer materials of different morphologies (fibres, fabric and films).

References:

[1] Nasef, M.M., Nallapan, M., Ujang Z. Polymer-based chelating adsorbents for the selective removal of boron from water and wastewater: A Review. Reactive and Functional Polymers 2014. 85, 54–68.

[2] Nasef, M.M., Guven, O. Radiation-grafted copolymers for separation and purification purposes: status, challenges and future directions, Progress in Polymer Science 2012, 37 (12) 1597–1656.