Optimization Studies on Hexavalent Chromium Removal using Novel Adsorbents

Abstract

ABSTRACT newlineChromium is one of the most widely used metals in industry. It is used in activities such as metal finishing, petroleum, power plants, and nuclear facilities resulting in large quantities being discharged into the environment. newlineChromium has emerged as one of the extremely toxic heavy metals whose pollution has raised a lot of concerns globally. Chromium exists in three principal oxidation states: metallic (elementary) chromium with oxidation state 0, trivalent chromium with oxidation state 3, and hexavalent chromium with oxidation state 6. Hexavalent chromium is recognized as a human carcinogen via inhalation. Workers in many different occupations are exposed to hexavalent chromium. Problematic exposure is known to occur among workers who handle chromate-containing products as well as those who perform welding, grinding, or brazing on stainless steel. In the present work, the adsorption of hexavalent chromium from aqueous solutions was studied in detail using adsorbents such as bentonite, chalk powder, cow dung ash, hydroxyapatite derived from various sources (batch studies), and Bt-Hap-Ca-alginate beads (packed column study). These adsorbents were characterized by XRD, FT-IR, SEM-EDS, and BET analysis and tested for their adsorption properties using synthetic wastewater prepared from K2Cr2O7 and electroplating industry effluent collected from Dobbespet, Tumkur district, Karnataka, India. The results suggested the involvement of electrostatic attraction, adsorption, and ion-exchange mechanisms in the adsorption of hexavalent chromium by these adsorbents. Page v newlineDesign of experiments (DOE) was used to determine model equations describing hexavalent chromium removal efficiency by the novel adsorbents with respect to the operating conditions such as pH, initial Cr (VI) concentration, adsorbent dose, and contact time. The optimum conditions for the adsorption of Cr (VI) using the novel adsorbents were determined using central composite design (CCD) of response surface methodology (RSM). Cow dung ash gave the