Effects of pH on adsorption of copper(II) onto ground peanut hulls

WCU Author/Contributor (non-WCU co-authors, if there are any, appear on document)
Stephen Adom (Creator)
Western Carolina University (WCU )
Web Site: http://library.wcu.edu/
Carmen Huffman

Abstract: This study explores the influence of surface charge on the adsorption mechanism of Cu(II) onto ground peanut hulls at different pH values. Ground hulls can be separated into two types based on their density. For both types of hulls, adsorption capacity increases with pH, but low density (LD) hulls adsorb more Cu(II) than high density (HD) hulls. The adsorption edge, which represents the pH range where a great increase in percent adsorption as a function of pH was observed, was lower for LD hulls than for HD hulls. For LD hulls, the maximum adsorption of copper (II) ions onto hulls was 79.1 % at pH 4 and the minimum adsorption was 65.2 % at pH 1 with a pH adsorption edge of 1.3-4.2. For HD hulls, the maximum adsorption was 72.3 % at pH 5 and the minimum adsorption was 64.6 % at pH 2 with a pH adsorption edge of 2-5.1. Measurements of pH before and after adsorption showed approximately no change in pH when the initial pH was less than 3.5 and a decrease in pH when the initial pH was > 3.5, suggesting a pH-dependent adsorption mechanism involving physisorption and/or chemisorption at low pH and cation exchange at pH 3.5 or greater.When pH was held constant using a buffer during adsorption, the maximum adsorption of Cu(II) ions to hulls was 99.8 % at pH 6 and the minimum adsorption was 10.9 % at pH 2 for LD hulls. For HD hulls, the maximum adsorption was 99.4 % at pH 6 and the minimum adsorption was 4.8 % at pH 2. A fit of the data to the Langmuir model gave maximum adsorption capacities of 14.6 and 11.43 mg Cu/g hull at pH 6 for LD and HD hulls, respectively. The increase in adsorption with pH observed in both studies is likely due to a reduction of competition between H+ ions and Cu(II) ions for binding sites on hulls at high pH values and also a more negative surface charge that binds positively charged Cu(II) ions at high pH values.To better understand the adsorption mechanism, the hull surface charge was examined. Acidimetric-alkalimetric titrations were performed at varying ionic strengths. The exact pHpzc could not be determined since acidimetric-alkalimetric titrations were not performed with high enough resolution. The value was however presumed to be between pH 4 and pH 11 based on the data acquired. The titration curves lacked variation with ionic strength which suggests surface charge on hulls may not be affected by ionic strength.For both hull types, surface charge calculated from the titration data increased with pH. Though this result was not expected, the peanut hulls are somewhat soluble in basic solution, which could explain the anomaly. Previous studies have shown that the addition of hulls to basic solution increases the pH, which makes the hulls appear to be negatively charged. A more likely occurrence is that an alkaline substance is being leached into the solution causing the pH to increase. Despite these complications, a local maximum of surface charge was observed at pH 3.5, which coincides with the pH for which the proposed adsorption mechanism shifts. This suggest that something unique is occurring at the hull surface at this pH.

Additional Information

Language: English
Date: 2020
adsorption, biosorption, copper, peanut hulls, pH, surface charge

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