Toxic Metals Removal From Municipal Solid Waste By Using Fungi
Municipal solid waste (MSW) has become the fastest-growing waste stream and its proper collection, recycling and final disposal is one of the most challenging and problematic tasks throughout the world. The World Bank report estimated that the generation rate of MSW was 0.34 kg/capita/day and 109,589 tones/day in 2005. In addition, these figures were expected to rise to 0.7 kg/capita/day and 376,639 tones/day by 2025. The MSW is commonly managed with methods, such as composting, landfill disposal, incineration and open dumping. Among the current management practices, the cheapest—open disposal—is the most common, particularly in developing countries, such as Syria. However, such practices are a potential threat to the surrounding environment. Rainfall percolating through these open dumping sites triggers biological processes, chemical reactions and physical changes as the MSW degrades, generating a highly contaminated liquid known as leachate. Leachate contains high concentrations of dissolved organic materials, inorganic substances and two categories of heavy metals: essential metals (copper, manganese, zinc and iron), and non-essential metals (cadmium, lead, mercury and nickel). These metals are among the major environmental pollutants because of their high toxicity. However, the exact composition of leachate can vary and depending on the type of waste, climatic conditions, rainfall patterns, hydrological factors and age of the disposal waste. Numerous physicochemical methods are commonly employed to remove heavy metals from effluent. These physical and chemical treatment processes are not effective for the management of MSW leachate. In fact, the less ecofriendly technologies are often inefficient, highly expensive and also possess secondary pollution. Hence, the development of new sustainable and ecofriendly technologies is essential for the treatment of leachate. Over the past few years, microbial treatment approaches have been attracting more attention, owing to their efficiency, comparative cost-effectiveness and eco-friendliness. These methods have many advantages, such as easy handling and maintenance requiring less technical support, higher metal uptake capacity with high treatment rates, minimal sludge production, and high reusability capabilities. The mechanism of removing cadmium through microorganisms is a highly complex process because there is competition among various substances, for surface binding sites. The metal biosorption mechanism depends on chelation, ion exchange, complexion, adsorption, absorption or micro-precipitation or a combination of these methods. The transfer of a metal from outside a cellular membrane into the intracellular space is a metabolism-dependent process governed by a living microbial system. This process is also a microbe’s active defence system, which is potentially capable of tolerating metals. Several microorganisms, including the fungi Pencillium, Rhizopus, Aspergillus and Mucor, as well as yeast and various bacteria, have been used in metal-polluted sites, owing to their continuous enrichment capabilities and highly adaptive nature. Fungi have a bulky biomass and many strains are capable of accumulating either a single heavy metal or multiple metals simultaneously. The heavy metals removal depends on several factors, such as pH, temperature, incubation period, and inoculum concentration. The protonation and deprotonation of functional and carboxyl groups play a significant role in the process of metal biosorption. Mosbah and Sahmoune (2013) stated that, the operational pH range could affect the accessibility of sorption sites on the cell wall surface and the capability of sorption sites to bind metal ions. Thus, these microbes could hypothetically be applied in situ in leachate treatment. Actual utilization of these strains would require that they have extraordinary tolerance to Cd2+: able to efficiently eliminate it. Yet, in spite of numerous studies on the ability of potential fungi isolates to extract Cd2+, the reaction of fungal strains to this metal has not been thoroughly studied. There my concern of this investigation is to assess the practicability of application indigenous fungus from MSW site and development fungal consortia (Trichoderma harzianum, Aspergillus niger and Aspergillus flavus) for the removal of cadmium metal from MSW leachate. The removal efficiency of cadmium metal under diverse conditions (pH, temperature, inoculum concentration and incubation period) should be examined.