Abstract
Adsorption kinetics of Ni, Cr, and Pb on gravels collected from constructed wetland was studied at varied metal concentrations and contact period for estimating the removal of heavy metals from wastewater. Batch experiments were conducted by shaking 120 ml of metal solutions having 5 concentration levels each of Ni (1.0, 2.0, 3.5, 5.0 and 6.0 mg l-1), Cr (1.0, 2.0, 3.0, 4.5 and 6.0 mg l-1), and Pb (1.0, 3.0, 6.0, 8.0 and 12.0 mg l-1) with 50 g of gravels for as function of time. Adsorption of Ni, Cr, and Pb on gravels ranged from 34.8 to 47.2, 42.7-54.9, and 47.5-56.9%, indicating their removal in the order: Pb > Cr > Ni. Freundlich model showed a good fit for Ni and Cr (R2>0.9) while Langmuir model fitted better for Pb (R2= 0.7). The pseudo-second-order model showed the best fit to simulate the adsorption rates of these metals on gravel.
TopIntroduction
In water scarce regions such as India, arid and semi-arid Africa, reuse of wastewater has become an important component of water resources planning. A number of issues related to wastewater management need to be understood and researched before reuse. Scientists around the globe are working on new ways of conserving water. It is an opportune time, to refocus on one of the ways to recycle water through the reuse of urban treated wastewater, for irrigation and other purposes and other ecosystem services which require lower water quality. It is a reliable and cheap source of nutrients but also contains heavy metals and other pollutants which contaminate soil-water-plant-animal continuum thus need removal. Conventional treatment plants for pollutants removal from wastewater are prohibitively expensive, generate huge quantities of toxic sludge and housed only in big cities (Vymazal, 2010). Decentralised constructed wetlands (CWs) are low cost and eco-friendly alternatives constructed wetland which removes more than 60% of the metals present in wastewater thus are suitable for rural areas and small towns (Marchand et al., 2010; Vymazal, 2010). A wide range of centralized sewage treatment methods are used instead in developing countries, including stabilization pond systems, septic tanks, activated sludges, trickling filters, anaerobic systems and land application systems (Canter et al., 1982). Kivaisi (2001) revealed that most developing countries have very few wastewater treatment facilities, mainly due to high costs of treatment processes and lack of effective environmental pollution control laws or law enforcement. Kivaisi (2001) showed constructed wetlands as the recently proven efficient technologies for wastewater treatment. Constructed wetlands are low cost (1% energy requirement, no chemical application, no skilled manpower requirement) and are easily operated and maintained, and have a strong potential for application in developing countries compared to conventional treatment systems. Alternative technologies to conventional wastewater management have been found to be environmentally sound and economically viable approach to wastewater management. Constructed wetlands have contributed to providing low cost sanitation in different countries. A limited database supports the capability of the subsurface flow wetlands process for effective removal of metals and other priority pollutants (Kovacic et al., 2006; Reed et al., 1987). It is thus important to establish through continued experimentation, the utility of constructed wetlands in the removal of heavy metals and other pollutants and the removal mechanisms involved. Keeping in view these observations, the objectives of the study are as follows:
- i)
to estimate the adsorption efficiency of a gravel materials and the removal time for different metals;
- ii)
to search the model which fits better the adsorption;
- iii)
to determine the intensity of adsorption and the types of bonds between metals and the substrate.
Key Terms in this Chapter
Kinetics: The branch of chemistry or biochemistry concerned with measuring and studying the rates of reactions.
Constructed Wetlands: Constructed wetlands are a sanitation technology that utilize natural removal mechanisms provided by plant vegetation, substrate (soil, gravel, sand), and associated microbial populations ( Maiga et al., 2017 ; Defo et al., 2017 ).
Adsorption: Removal process in which there is adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent.
Heavy Metal: Any metal with a specific gravity of 5.0 g/cm 3 or greater, especially one that is toxic to organisms. “Heavy metals” would then refer to (1) transition elements; (2) rare earth elements, which can be subdivided into the lanthanides and the actinides, including La and Ac themselves; (3) a heterogeneous group including the metal Bi, the elements that form amphoteric oxides (Al, Ga, In, Tl, Sn, Pb, Sb and Po), and the metalloids Ge, As and Te (Appenroth, 2010 AU99: The in-text citation "Appenroth, 2010" is not in the reference list. Please correct the citation, add the reference to the list, or delete the citation. ).
Adsorbate: A substance that is adsorbed.
Adsorption Isotherm: When an adsorbent and adsorbate are in contact for long period of time, an equilibrium is established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by adsorption isotherms.
Adsorbent: A substance that adsorbs another material.
Adsorption Capacity: The amount of adsorbate taken up by the adsorbent per unit mass (or volume) of the adsorbent. The adsorption capacity of a solid desiccant for water is expressed as the mass of water adsorbed per mass of desiccant.