Design and Optimization of Heavy Metal Chromium (VI) from Aqueous Solutions by Tamarind Fruit Shell Activated with Sulfuric Acid

The industries of leather, plating, metal processing, wood production, painting, textile, steel making and bottling are the main industries for pollution of heavy metal chromium. Chromium in the two main oxidation states consists of chromium (VI) and chromium (III). Chromium (VI) is more dangerous, more cancerous and toxic for living organisms compared with chromium (III). In addition, low chromium (VI) concentration can cause health problems, including damage to the liver, as compared with chromium (III). In addition, lower concentrations of chromium (VI) can cause health problems including liver damage and skin cancer. Acceptable limits of chromium (VI) for discharging to surface water and drinkable water are 0.1 and 0.05 mg L-1, respectively. Therefore, wastewater treatment for reaching the desired level of chromium (VI) is essential. There are several methods including chemical oxidation-reduction, ion exchange, electro dialysis, electrochemical treatment, evaporation, solvent extraction, reverse osmosis, chemical treatment and adsorption to reduce chromium (VI) from aqueous and sewage solutions. The main drawback of many of these methods is the high operating cost and remaining sludge disposal problems. Among the techniques, the adsorption method is preferred because of its simplicity, medium-performance and economic conditions. In the adsorption process, the physical and chemical properties of adsorbent are very important for achieving high adsorption efficiency. In the present study, activated particles with sulfuric acid were used to improve the removal of chromium (VI) from aqueous solution. In the adsorption process, the removal of chromium (VI) metal depends on several process variables in a discrete system such as the initial concentration of chromium (VI) solution, adsorbent dose, solution pH, vibrational speed and contact time. Optimizing process variables is necessary to achieve the maximum process efficiency for removing pollutants. A laboratory statistical design approach is necessary to reduce the number of experiments, create an appropriate model for process optimization and also evaluate the effect of response variability. Recently, several types of test design methods have been used to optimize the multivariate chemical process. Response surface of methodology is a set of statistical and mathematical methods for designing experiments in this field. Nevertheless, no study has been carried out on the optimization of the removal of chromium (VI) by particles of the shell modified with sulfuric acid. Chromium (VI) solution was prepared by dissolving potassium dichromate (K2Cr2O7) in distilled water. It should be noted that the dominant form of chromium solution in terms of pH and chromium (VI) contaminant concentrations is (HCrO4) – and (CrO4) 2- in this study. Preparation of modified tamarind fruit shell: Tamarind fruit shells were prepared from the central part of Iran (Isfahan province). The shells were washed with distilled water and placed in an oven at 110 °C for 4 hours. The dried shells were crushed and then passed through a filter with a size of 200 microns. The resultant particles were contacted with concentrated sulfuric acid (98 % w/w) for chemical activation and H+ ion saturation in a 1: 1 weight ratio and in an oven at 150 ° C for 24 hours. The particles were then washed with distilled water and placed in a 1% sodium bicarbonate solution for 2 days. The material was then washed with distilled water and dried at 100 ° C for 5 hours. With this activation operation, H+ ions are located on the adsorbent surface and increase the adsorption of chromium samples. In this study, chromium samples are more (HCrO4) - and (CrO4) 2- which have an electric charge. These negative samples are definitely adsorbed to the shells by placing H+ ions on the surface of the adsorbent. Adsorption -Tests: The effect of 5 independent variables on chromium (VI) adsorption of aqueous solution was investigated by the particles of tamarind fruit shells in a discrete medium. All experiments were performed in accordance with the D-optimal matrix method. To adsorb chromium (VI) by tamarind fruit shells, different weights of the tamarind fruit shell (1-7 gr/L) were combined with 50 ml of chromium (VI) solution and different concentrations (50-150 mg/L). The chromium aqueous solution (VI) was prepared by dissolving preset amounts of K2Cr2O7 powder in distilled water. The pH of the solution was adjusted by a solution of 1 M hydrochloric acid or 1 M sodium hydroxide over the range of 1-10. The residual concentration of chromium ions (VI) in the soluble phase was determined by spectrophotometer detection of UV rays at 540 nm by the formation of diphenyl carbazide color. Adsorption Capacity (mg g-1) of the bimetallic system was calculated as follows: where C0 and Ce are the primary and final concentrations of chromium (VI) (mg L-1), respectively. V and W represent the volume and mass of the solution containing the tamarind fruit shell particles, respectively. The adsorption of heavy metal chromium (VI) by the modified tamarind fruit shell as a sorbent was studied by changing the pH of the solution, the adsorbent dose, the initial concentration of chromium (VI), contact time and vibrational velocity (shaker). The design of the D-optimal experiment, along with the response surface procedure modeling, was used to maximize the removal of chromium (VI) from the aqueous solution by the particle of the tamarind fruit shell.