(347o) Adsorption Characteristics of Hexavalent Chromium from Aqueous Solution Onto Biochars

Chromium contamination of water is a growing concern due to its toxic nature. Chromium can exist in two major oxidation states: trivalent and hexavalent. Hexavalent Chromium (Cr(VI)) is more toxic than trivalent Chromium (Cr(III)). The allowable limit of Cr(VI) in surface water is 0.1 mg/L and in potable water it is 0.05 mg/L. High concentration of Cr(VI) can cause severe human health consequences. It is therefore important to remove Cr(VI) from wastewater before disposal. Various methods have been used for Cr(VI) removal including precipitation, ion exchange, membrane separation and adsorption. However, most of these treatment methods have limitations such as poor removal efficiencies, high operational and maintenance cost, or toxic sludge production. Recently, biochar has received considerable attention as an adsorbent because of its high pollutant removal efficiency, low cost, and availability.

This study focuses on the synthesis of biochars from wood shavings, sugarcane bagasse, and goat manure and their application to remove Cr(VI) from aqueous solution. The biomasses were pyrolyzed in a Non-stirred Parr Pressure Vessel at 450°C to synthesize biochars. All biochars used consisted of micrometer sized particles. Biochars were characterized using Scanning Electron Microscopy (for morphology), Brunauer-Emmet-Teller nitrogen physisorption method (for specific surface area) and Total Organic Carbon Analyzer (for carbon content). SEM micrographs reveal that biochars are amorphous and porous. Surface areas of wood shavings, sugarcane bagasse, and goat manure were determined to be 169.3 m²/g, 66.7 m²/g, and 68.3 m²/g, respectively whereas carbon contents were found to be 78.3%, 73.8% and 53.9%, respectively. Batch tests were conducted to evaluate the adsorption characteristics of Cr(VI) onto biochars by varying pH and contact time. Tests were performed at pH values of 2, 5 and 9 for a biochar dosage of 10 g/L on all tests. The tests were continued for 5 days and samples were collected at each day to determine the residual Cr(VI) concentration using the Graphite Furnace Atomic Absorption Spectrophotometer. Fourier Transform Infrared (FTIR) spectroscopy was also used to explore the changes of surface functional groups on biochars.

The results indicate that both sugarcane baggase and goat manure biochars are highly effective (90%) in removing Cr(VI) although at different pH values. For sugarcane bagasse it is pH-9 whereas for goat manure it is pH-2. Woodchip biochar is not effective at all in Cr(VI) removal (30%). The general characteristics for Cr(VI) removal as a function of time are observed to be similar in all cases. The adsorption increases rapidly in the first 2 to 3 days, then increases asymptotically to the maximum value over the next few days. FTIR results indicate insignificant changes in prominent functional groups before and after its use in removing Cr(VI) confirming that physical adsorption has taken place. The FTIR spectra reveal the presence of C-O, C=O, and O-H functional groups on the surface of biochars derived from woodchips, sugarcane bagasse and goat manure. In addition, C=C stretch was found on the surface of the biochar derived from woodchips and sugarcane bagasse whereas C-C triple bond stretch was observed on the surface of the biochar derived from goat manure. Hence, biochars derived from sugarcane bagasse, and goat manure can be promising adsorbents for Cr(VI) removal from water contaminated with Cr(VI).