N-Methylation of Pyridinic-N on Flame Resistant PAN Fiber with Methyl Iodide to Form Quaternary Nitrogen (N-Q) Adsorption Sites for Nitrate Removal from Aqueous Solution

Pyridinic nitrogen (N-6) in flame resistant polyacrylonitrile (PAN) fiber was converted to positively charged quaternary nitrogen (N-Q) of methyl pyridine using methyl iodide at 40°C to prepare carbon fiber adsorbents to remove a negatively charged pollutant of nitrate from aqueous solution. The prepared PAN fiber adsorbent could uptake nitrate above 0.3 mmol/g from 3 mmol/L nitrate solution, whereas only 0.03 mmol/g of nitrate was captured with the unmodified PAN fiber. Some reaction conditions for methylation were examined to maximize nitrate and above 0.6 mmol/g of nitrate adsorption could be achieved at equilibrium solution pH (pHe) 3. Langmuir adsorption isotherm could be applied to the nitrate adsorption at solution pHe 3 and calculated adsorption affinity (Ke) and capacity (Xm) of nitrate were 0.24 L/mmol and 2.39 mmol/g, respectively, for the optimum prepared conditions for the adsorbent. Influence of pHe on nitrate adsorption was also inspected for the modified PAN adsorbent and above 0.3 mmol/g at pHe 1-3 was maintained and above 0.15 mmol/g in a constant value at pHe 6-10 under standard conditions indicating that 0.15 mmol/g or greater number of quaternary nitrogen (N-Q) might be exposed to the PAN surface. XPS N1s analysis and Boehm titration support the presence of N-Q species on the modified PAN sample to adsorb nitrate.