Wastewater released from industrial activities is loaded with heavy metals such as nickel, arsenic, lead, zinc, copper, cadmium, chromium. The current investigation is conducted to remove the hexavalent chromium (Cr VI) from the simulated wastewater by adsorption process using activated carbon. Experimental studies were carried out in batch and continuous mode. For studying the fixed bed, namely bed height, concentration and flow rate at the inlet were varied. In batch studies, as the initial concentration of the sample, contact time of adsorption and adsorbent dosage for the given initial concentration increases, adsorption also increases. The optimal pH value is in the acidic range. Isotherms were plotted and studied to understand the adsorption. For fixed bed, increasing concentration decreases the breakthrough time. Experiments show that the time required for exhaustion reduces when there is increase in flow rate and concentration. Fixed bed adsorption results were fitted to the models like Yoon-Nelson and Thomas.
Top1. Introduction
Rapid industrialization and technological advancements result in heavy waste water pollution which is a global concern due to disposal of elements like heavy harmful metals such as copper, zinc, nickel, chromium, lead and iron into the water streams (Kulkarni et al., 2018). Pollution of lakes, rivers and other water streams is caused by the release of heavy harmful metals in them which is a huge threat to not only to human beings but also to the flora and fauna of various water bodies. In high dosage, chromium and lead are carcinogenic in nature which creates a risk for human health and flora and fauna of water bodies. Lead, which is a heavy metal, is classified under toxic category while other heavy metal i.e., chromium is classified under micronutrient category and thus, intake of even small concentration of chromium is lethal to the people’s health. Heavy metals are introduced in the human body via various ways like drinking water, animal feed, pharmaceutical medicines, green fodder, etc. These heavy metals can be accumulated in the human body directly or indirectly via the food chain. These toxic metals can enter drinking supplies as the industrial waste water is introduced in the water streams. According to WHO, chromium concentration in industrial wastewater should be 1 mg/l maximum. Excessive uptake of chromium in the human body results in capillary damage, irritation to central nervous system, gastrointestinal irritation and hepatic and renal damage. These metals can get accumulated in the soil causing the agricultural damage. In drinking water, concentration of lead and chromium must be 0.05 mg/l at maximum which is recommended by the World Health Organization (WHO). Because of all these harmful effects, it is extremely necessary to remove these toxic heavy metals before releasing them into the environment. For removal of heavy metals, various methods like reverse osmosis, electro floatation, ion exchange, chemical precipitation, electro dialysis, membrane separation, biological separation, adsorption, etc. are employed. From these different methods, we will use the adsorption method.
During the recent years, the usage of adsorption for treatment of waste water has become more prevalent due to its effectiveness and cost-effectiveness for removing pollutants (heavy metals) from waste water. Chromium has broad applications in different industrial processes. The inappropriate disposal of chromium from various industrial sources leads to the pollution and it has adverse effects. The adverse effects of chromium on the plants, humans, environment and animals include mutagenic, carcinogenic effects, tissue damage and teratogenic effects (2). Cr(III) and Cr(VI) are the two states of chromium ions known as trivalent and hexavalent state respectively. Cr(III)can be used as an important trace element. On the other hand, Cr(VI) is non-essential and toxic in nature which causes various diseases like dermatitis, gastric and kidney damage, lung cancer, respiratory tract and eye irritation (Mitra et al., 2017). Therefore, it is necessary to reduce its concentration in waste water before getting released into the aquatic system and getting included in the food chain. The conventional processes used for chromium removal include Cr(VI) immobilisation or reduction of toxic Cr(VI) to less toxic Cr(III). Some methods for removal of hexavalent chromium involve methods like solvent extraction, membrane filtration, electrokinetic procedures and leaching (Mitra et al., 2017). The mentioned processes are of high cost. Thus, adsorption of chromium on the various adsorbent materials like activated carbon, titanium dioxide, goethite, zeolites, etc. is a low-cost method which has design flexibility and also easy to operate.
Adsorption is a surface phenomenon. In this mass transfer process, molecules get accumulated on the surface of another slid surface. It can be also stated as a physical or chemical bonding of molecules on the surface(active sites) of material through Van der Wall forces or by chemical bonding on the surface with the active sites (Amal, 2018). In adsorption, adsorbate is a material which gets attached to the surface of adsorbent. Adsorbate is the solid particle or molecule which is going to adsorb on another surface i.e., effluent particle while adsorbent is the solid surface on which molecules are going to adsorb; e.g., carbon, zeolite, porous clays, etc. For the treatment of waste water, the commonly used adsorbent is activated carbon as it has ability of adsorbing various organic compounds.