Reduction of hexavalent chromium to trivalent chromium using green technology

Hexavalent Chromium (Cr⁶⁺) is carcinogenic and harmful substance that can harm human health. These Cr⁶⁺are mainly from electroplating industry that produce metal with have high corrosion resistance to environmental conditions. Thus, the residue from this industry generate chromium waste that can pollute the environment if it is not treat properly. Common method in treating Cr⁶⁺is by using Sodium Metabisulfite (SMBS) as reducing agent to reduce it to Cr³⁺ and further process in hydroxide precipitation for completely remove the chromium content in the wastewater effluent. This chemical generated harmful gases which are Hydrogen Sulfide (H₂S) and Sulfur dioxide (SO₂) that give health problems for human through inhalation. Production of the huge amount of sludge that generates from the treating process using SMBS, eventually will be disposed at landfill gives effect on the environmental aspects. Other inorganic waste of that can be used as a reducing agent is Leeden Gas sludge (LG sludge) that are from acetylene production gas. LG sludge contains high in Calcium (Ca) element and Ferum (Fe) ions that are essential for the reduction of Cr⁶⁺ to Trivalent Chromium (Cr³⁺) The analysis of LG sludge characterization was done by XRF and XRD instrument with the result of high Ca and Fe ions with Calcium Hydroxide (Ca(OH)₂) alkaline properties. The initial concentration of Cr⁶⁺ is 3.25 mg/L using HACH Method 8023 and the initial total Cr concentration is 1124 mg/L was analyzed by using ICP-AES. From the jar test result in the effect of pH variation (from pH 2.5, 3.5, 4.5, 5, 5.5, 6, 6.5, 7.5, 8, 9 to 10), the highest % reduction is at pH 5.51 with 61.5% reduction and the dosage of LG sludge was 1.15 mL. The ORP value was at 320.2 mV with 1.25 mg/L of Cr6+ concentration. Next, result from the effect of volume LG sludge variation (from 0.7 mL, 0.8 mL, 0.9 mL, 1.0 mL, 1.1 mL, 1.2 mL, 1.4 mL, 1.6 mL, 1.8 mL, 2.0 mL to 2.1 mL), 100% reduction occurs at pH 6.04 with the dosage of 1.8 mL and the ORP value was at 264.9 mV. The result of SMBS dosage as a reducing agent was 10.5 mL for 100% reduction. Comparison between the dosage of SMBS and LG sludge is that SMBS used 8.7 mL more than LG sludge. Thus, the optimization of cost can be accomplished by replacing the SMBS with LG sludge in treating the Cr waste. Furthermore, LG sludge can also act as precipitate agent due to the formation of brownish precipitate at the bottom of the beaker which suspected to be Cr(OH)₃ precipitate. Other than that, LG sludge can also be used as pH adjustment without adding any other chemical due to it's high Ca properties which is alkaline. LG sludge has better performance of reducing the Cr⁶⁺ to Cr³⁺ compare to SMBS as reducing agents.