The adsorption of nitrate, chromium (VI), arsenic (V) and selenium (VI) anions in an amine modified coconut coir (MCC-AE : with secondary and tertiary amine functionality) were studied to determine the capability of this easily prepared and low-cost material in removing typical groundwater anion contaminants.
Batch adsorption-ion exchange experiments were conducted using 200 mg MCC-AE, initially containing chloride as the resident anion, and 50 ml of different anion-containing water of varying concentrations. It is presumed, at this low pH, that only SeO42− remained as a divalent anion, while monovalent species H2AsO4− and HCrO4− predominated in their respective exchanging ion solutions. The adsorption data were fitted using the Freundlich equation and maximum adsorption for each anion was estimated using their respective Freundlich equation constants. MCC-AE exhibited preference for divalent Cr (VI) and Se (VI) anions compared with the Cl− resident ion. Maximum As (V) adsorption was 0.086 mmol/g, while maximum adsorption of Cr (VI), NO3− and Se (VI) anions was 0.327 mmol/g, 0.459 mmol/g, and 0.222 mmol/g, respectively. The ion exchange capacity of MCC-AE is estimated, based on its exchange capacity for nitrate, to be within 0.46 mmol of positive charges per gram. Similar adsorption experiments were conducted for comparison using commercial chloride-form Amberlite IRA-900 strong base (quaternary amine functionality) anion exchanger, with an exchange capacity of 4.2 meq/g. Maximum adsorption of the different ions in IRA-900 was about 3 times higher for NO3−, 9 times higher for Se (VI), 10 times higher for As (V) and 9 times higher for Cr (VI), than that in MCC-AE. Differences in the ion exchange behavior of MCC-AE and IRA-900 were probably due to the different amine functionalities in the two exchangers. The results suggest that MCC-AE may be used as a low-cost alternative adsorbent/ion exchanger for treatment of anion contaminants in groundwater.