Agricultural Waste Absorbents for Heavy Metal Removal

Abstract

Large numbers of toxic and poisonous chemicals are released in our surroundings from different industries resulting in contamination of aqueous medium, atmosphere, and geosphere at an alarming level. Aqueous medium pollution that results from heavy metal has come into being as a severe environmental issue. Some poisonous metals include zinc, mercury, lead, chromium, cadmium, nickel, and copper, which are of particular concern in curing industrial wastewater. They are non-biodegradable in nature, make aggregate in living bodies and most of them are cancer-causing agents. A number of extensive industrial researches have been conducted for the safe removal of pollutants and metallic elements from industrial effluents and wastewater. There are many different ordinary treatment strategies that have been utilized to treat these aqueous phase toxins, such as flotation, ion-exchange, coagulation, reverse osmosis, electrolysis, and complexation. The adsorption is more widely accepted, and more promising techniques can be utilized to eliminate certain types of pollutants by selecting appropriate adsorbent. It offers many advantages over other conventional methods and is accepted as one of the productive, effective, labor-saving, valuable, and reasonable methods. There are large number of adsorbents that have been used to adsorb heavy metals from aqueous media such as activated carbon, activated alumina, clays, zeolites, polymer clay-based nanoadsorbents, metal oxide-based nanoadsorbents, graphene, and carbon nanotubes; some other absorbents including chitosan, lignin, cellulose, starch, and fungi have also been used for safe removal of heavy metals. Agricultural wastes come from the growing and initially processed agricultural stuff, including organic products, vegetables, fruits meat, and dairy items, which nowadays have been used as economic and environment-friendly adsorbents. Agricultural wastes especially those that contain cellulose have a high biosorption capacity. Adsorbents from plant wastes can be used with or without modification for the reduction of a pollutant from aqueous media. Generally, chemical modification of plant wastes enhances their adsorption capacities as compared to unmodified. Rice husk in pristine form and in activated rice form is utilized for chromium (VI) removal. Different kinds of agricultural waste materials including peanut shells, soybean hulls, rice straw, bagasse, sugarcane, and walnut shells have been applied for lead adsorption. In some studies, rice and wheat bran were analyzed for cadmium adsorption, and excellent results were reported. Other studies showed the application of rice husk and rice polish in both pristine and composite forms for effective cadmium adsorption. The bark of some plants including Picea glehnii and Abies sachalinensis and biomass of dried plants (Parthenium) were checked for cadmium removal. Adsorption experiments performed using hazelnut shells, green shell, walnut shells, and peanut hulls exhibited good results toward cadmium adsorption. Some other adsorption studies conducted using activated carbon prepared coir pith, bagasse pith, and dates and peanut shells. Their cadmium adsorption capacity was found to be up to 98%. One study showed that tea-based adsorbent reduced almost 70% of the hexavalent chromium into Cr(III). In short, agricultural waste can be used as low-cost and most beneficial adsorbent for removal of heavy metals from aqueous media.