Removal of Hexavalent Chromium from Contaminated Water by Chinese Herb-Extraction Residues

Hexavalent chromium (Cr(VI)) in wastewater is a great risk to the quality of water sources and to human health. Meanwhile, Chinese herb-extraction residues (CHER) are the by-products from the extraction process of Chinese medicine. This study investigated the use of CHER to adsorb Cr(VI) from aqueous solution. Results indicated that adsorption was maximum at solution pH of 2.0 while the Cr(VI) removal efficiencies for chuanxiong rhizome residue (CRR) and Chinese wolfberry residue (CWR) were 86.56 and 99.93%, respectively. Isotherm data were modeled by using Freundlich, Langmuir, and Temkin isotherms. The maximum monolayer adsorption capacities for CWR and CRR were 79.60 and 36.21 mg g(-1), while their corresponding adsorption capacities obtained from experimental data were 37.30 and 32.42 mg g(-1), respectively. However, Freundlich isotherm fitted the data well. Both adsorption and reduction of Cr(VI) to Cr (III) forms followed by complexation onto the adsorbent surfaces favored removing Cr(VI). The results also suggested that the abundant and cheaply available CRR and CWR can be used as efficient adsorbent materials for Cr(VI) removal from wastewater.