Binod K. Chaudhary

Chemical and Environmental Engineering / College of Engineering

Understanding synthesis and regeneration of hybrid fiber-ferric hydroxide adsorbent for arsenic removal
This research investigated the modification of homopolymer polyacrylonitrile (PAN) fibers for use as an adsorbent for removing arsenic from drinking water.  The fibers were chemically modified and crosslinked using combinations of hydrazine hydrate and sodium hydroxide before being loaded with ferric hydroxide using two different iron loading procedures.  Arsenate adsorption was a function of both the iron loading and the properties of the underlying fiber.  The best arsenate removal performance was obtained using the simplest pretreatment procedure of soaking in 10% NaOH at 95 °C for ninety minutes, followed by precipitation coating of ferric hydroxide.  This suggests that adsorbents based on inexpensive homopolymer PAN fabric may be produced in developing areas of the world where commercial products may not be available.
To understand arsenic reactions with ferric hydroxides, density functional theory modeling was employed to calculate the reaction energies and activation barriers for three modes of arsenate adsorption to ferric hydroxides.  Physical adsorption of arsenate to ferric hydroxide proceeded with no activation barrier.  The conversion of physically adsorbed arsenate into monodentate surface complexes had activation barriers ranging from 62 to 73 kJ/mol. Similarly, the activation barriers ranging from 79 to 112 kJ/mol were found for the conversion of monodentate surface complexes to bidentate, binuclear complexes.  For release of arsenate from uncharged bidentate complexes, activation barriers as high as 167 kJ/mol were encountered. This showed that the slow kinetics associated with arsenic adsorption and desorption can be attributed to the high activation energies for forming and breaking bonds with the ferric hydroxide. For understanding the effect of support properties on arsenic regeneration from ferric hydroxide loaded adsorbents, solutions containing only 0.10 to 5.0 M NaOH or 0.10 to 1.0 M NaCl, as well as solutions containing both compounds, were used as regenerants.  Adsorbents containing support material with no anion exchange functionality could be regenerated with NaOH solutions alone.  Regeneration of media containing strong base anion (SBA) exchange functionality was greatly enhanced by addition of 0.10 M NaCl to the NaOH regenerant solutions.  On all media, 5 to 25% of the arsenate was resistant to desorption in 1.0 M NaOH solutions.