Low-Cost Sorptive Chromium Removal To Minimize Environmental Pollution From Chromium-Laden Tannery Effluent

Abstract

The growing demand for leather goods necessitates the establishment of additional tanneries as well as an increase in the production rate of those that already exist. Despite numerous alternatives, chrome tanning remains the most widely used method, with 85 to 95% of all tanneries using it. Wastes from chrome tanning cause environmental and human health problems. At low levels, Cr(VI) is dangerous to humans and other species, and chromium disrupts river ecology by impairing their ability to self-purify. A critical first step in chromium remediation is reducing chromium at the source. One of the primary approaches is increasing chromium uptake within the tanning industry. When possible, replacing chromium with alternative compounds is an intriguing substitute that needs to be studied further. The chromium that left the industry's compound, the remaining concentration, which may be oxidized to Cr(VI) should be reduced to the level it cannot pose environmental and health impacts. Adsorption, a method with cheap installation and operational costs, good efficacy, and adaptability, especially in low-income nations are comparatively preferable. Thus, the determination of the distribution and speciation of chromium in water, soil, and parts of edible plants in tannery-contaminated areas of Modjo city and the investigation of the adsorption capacities of natural and treated locally available zeolite, pumice and coffee husk biochars were conducted under laboratory experimental setups. Twelve, water and soil samples from affected locations, and 6 samples of different edible plants growing near tanneries, were collected using a conventional approach to determine chromium contamination in the environmental samples. The distribution and speciation of chromium in water, soil, and parts of edible plants in tannery contaminated areas of Modjo city show a clear indication of chromium mobility in the environment. Due to the ease of effluent disposal and water consumption, most tanneries in Modjo city are constructed nearby Modjo River. Besides home uses, the river water, which receives chromium-containing wastewater from the tanneries, is also used for the urban gardening of food plants. In the upstream section (at 10 meters from the tannery discharge point) of the Modjo River, the total chromium was 20.6 mgL-1 which dramatically reduced (r = -0.93, p < 0.05) to 0.126 mgL-1 in the downstream section (800 meters from the first upstream point). The Cr(VI) concentration ranges from 0.23 ± 0.032 to 2.82 ± 0.02 mgL-1 , with a statistically negligible drop (r = -0.76, p > 0.05) as moving downstream. The concentrations of Cr(III) and Cr(VI) in the soil were 2.78 ± 0.37 and 4.57 ± 1.01 mg kg-1 , respectively, which are higher (p < 0.05) than the control (0.18 and guideline levels. In the edible plants, chromium concentrations ranged from 7.98 ± 0.63 mg kg-1 for green pepper to 14.45 ± 0.34 mg kg-1 for XIV carrot, with an order of carrot > beetroot > lettuce > cabbage > tomato > green pepper. Chromium levels in the contaminated plants were substantially higher (p<0.05) than in the control region, which ranged from 0.14 mg kg-1 for lettuce to 0.31 ± 0.01 mg kg-1 for tomatoes. It has also been proven that plants' roots accumulate more chromium than their leaves and fruits. Chromium concentrations determined in the tannery water and soil, and edible plants were greater than the required quantity for a healthy environment and human consumption. Three sets of biochars were produced from coffee husks as a feedstock at 3500C (B-350), 5000C (B-500), and 7000C (B-700) pyrolytic temperatures to choose suitable pyrolysis temperatures for the potential biochar to remove Cr(VI). To investigate the properties of the biochars produced, proximate analysis, XRD, and FTIR tests and a batch Cr(VI) adsorption experiment were performed. B-350 had the highest yield, while B-700 has the largest surface area but fewer functional groups. B-500 has a moderate surface area and many functional groups, comparatively. At 25 mgL-1 initial Cr(VI) concentration, pH 2, 1 g biochar, and 60 min contact, the maximum Cr(VI) percent removal and capacity achieved were onto biochar pyrolyzed at 5000C (B-500). The Langmuir isotherm governs adsorption, and it follows a pseudo-second order reaction model, which describes the chemisorption process. For B-500, the equilibrium adsorption capacity was 23.11 mgg-1 . It is determined that, while a greater pyrolytic temperature provides more surface area biochar, the destruction of C, H, and O-based functional groups, which are necessary for Cr(VI) adsorption, limits the amount of Cr(VI) adsorbed. Biochar produced at a moderate temperature has an optimal surface area for adsorption, as indicated by its greater adsorption capability. Similarly, the potential of aluminum-coated zeolite and pumice for removing Cr(VI) from an aqueous solution was investigated and compared to their natural forms in this study. The physical and chemical properties of natural zeolite (ZE), natural pumice (PU), aluminum coated zeolite (ZE-A), and aluminum-coated pumice (PU-A) were investigated using the volumetric method, N2-surface area analyzer, X-Ray Fluorescence Spectrometry (XRF), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), X-Ray Diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR), respectively. A batch adsorption experiment was conducted at a varying pH, adsorbent dosage, contact time, and the initial Cr(VI) concentration. The optimum pH for Cr(VI) adsorption onto the adsorbent was discovered to be 4, with a 20 mgL-1 initial Cr(VI) concentration, 0.1 g dose in 120 min contact time. The Cr(VI) removal was 56.87 and 64.58% for ZE and PU, respectively, which was XV improved to 84.34 and 91.64% for the aluminum coated, ZE-A and PU-A. The Langmuir adsorption isotherm is the best fit, with R2 values of 0.992, 0.992, 0.994, and 0.993 for ZE, ZE A, PU, and PU-A while the equilibrium adsorption capacity (qe) is 11.37, 16.87, 12.92, and 18.33 mgg-1 for ZE, ZE-A, PU, and PU-A, respectively. The adsorption of Cr(VI) onto natural and aluminum-coated zeolite and pumice are best described by the pseudo-first-order kinetic model. Zeolite and pumice are both inexpensive and widely available materials that can be utilized as adsorbents. However, the considerable increase in the adsorption capacity of aluminum-coated zeolite and pumice (r 2 = 0.72, p<0.05) shows intriguing potential for chromium removal from wastewater in impoverished nations. Wastewater containing chromium discharged from tanneries significantly pollutes the water, soil, and edible plants in the tannery’s surroundings. The presence of Cr(VI) in this environment confirms the possible exudation of Cr(III) into mobile and toxic Cr(VI). Biochars produced at the optimum pyrolytic temperature from waste coffee husks under optimum adsorption parameters demonstrated effective Cr(VI) removal capacity and can be used in developing countries that cannot afford conventional treatments. Furthermore, natural rock media abundant in Ethiopia's Rift Valley that has been modified with aluminum can remove Cr(VI) from tannery discharges. The results of the study can be used for further research into adsorbent materials for practical use in reducing Cr(VI) to an acceptable level should focus on detailed characterization and pilot studies using wastewater from leather tanneries.