STUDIES ON THE USE OF ACTIVATED CARBON DEVELOPED FROM A FERTILIZER WASTE FOR THE REMOVAL OF ORGANIC AND INORGANIC POLLUTANTS

Abstract

Increase in economic and population growth in developed areas have led to the need for greater quantities of water than are readily available. At the same time, the discharge of partially treated waste waters in ever increasing volumes has led to the degradation of water ways. When waste water is recycled, rather than discharged, it becomes a resource and treated effluent a high quality water that can be reused. Activated carbon adsorption is an important process used to remove' pollutants from waste waters. The drawback of this process is the high cost. involved in its manufacture and regeneration and hence it is necessary to look for other economically feasible adsorbents. The primary objective of the present research is to prepare a low cost carbonaceous adsorbent from the waste slurry of fertilizer plants and to evaluate the chemical and physical parameters associated with the adsorption of some phenols and heavy metal ions on this adsorbent material. The overriding theme is directed towards understanding the adsorption process. The ions of some of the heavy metals are toxic pollu tants. Most of these are cumulative poisons capable of being assimilated, stored and concentrated by organisms that are exposed to low concentrations of the metals,or their compounds, for long periods or repeatedly. Eventually, the buildup of the metal in the tissues is sufficient to cause noticeable -11- physiological effects. The heavy metals :' chosen for study are cadmium, chromium, copper, cobalt, lead, mercury, molybdenum nickel and zinc. Phenol and its derivatives occupy a prominent position on the priority list as organic pollutants. Phenolic compounds besides being a potential health hazard also impart bad smell and taste to water even when present in traces. The phenols studied are phenol, 4-nitrophenol, 2y4-dini trophenol, 2y V6-trinitrophenol,' 4-chlorophenol,1 U 3-di hydroxybenzene, l,/3y'5-trihydroxybenzene and a-naphthol. The waste material obtained from National Fertilizers Limited,.' Bhatinda( India) has been converted into activated carbon. The waste slurry has been activated by treating it first with hydrogen peroxide, to oxidise the adhering organic matter, then heating at 450°C in the presence of air. The effect of environment on the surface properties of carbon generated from waste slurry has been studied by activating the waste material in steam and nitrogen atmos pheres. The three samples have been characterized and compared on the basis of surface area and adsorption capacity. The one activated in air shows best sorption property and hence has been used in all further investigations. The extent of adsorption at equilibrium is dependent on the physical and chemical characteristics of the adsorbate, adsorbent and the experimental system. Laboratory scale -illbatch studies have been performed to establish optimum condi tions for the adsorption of phenols and heavy metal ions. The parameters evaluated include hydronium ion concentration, temperature and adsorbent dose etc. The effect of surfactant has also been studied. It is found that the adsorption of phenols is an endothermic process and the uptake increases with temperature. All the phenols are strongly adsorbed at pH below their dissociation constant value (pK ). A direct relationship a is observed between pK value and percentage adsorption. a The various substituents on the phenyl ring of the phenols also influence the adsorption of these compounds. The inter action of the aromatic ring of ... phenols with the surface of • active carbon has been considered to be the major influence on the adsorption process. The charge-transfer interaction as postulated by various workers is supported on the basis of the electron densities in the sorbate mole cules. As the positive charge on the benzene ring increases the formation of donor-acceptor complex between phenol and carbon is facilitated. Hence the phenols in which an electron withdrawing group (-NO ) is the substituent,are more adsorbed in comparison to one in which an electron donating group (like-OH) is present. Addition of an anionic surfactant to phenol solution does not affect the adsorption of phenol in any.way. -IVThe adsorption of heavy metal ions does not follow any set pattern. Chromium, mercury and lead are highly adsorbed, molybdenum is fairly adsorbed but the uptake of copper and cadmium is quite low while the adsorption of zinc, nickel and cobalt is negligible. The uptake of all the metal ions is found to decrease with temperature. The optimum pH is found to vary for various metal ions and it is found that the adsorption is maximum at a pH which preceeds hydrolysis. Competitive adsorption from a mixture of metal ions has also been studied. The uptake of Pb + , Hg +and Cr + in the presence of each other presents interesting results. Chromium is not effected by the presence of mercury while mercury adsorption decreases in presence of chromium ..Lead and mercury also show a slight decrease in their adsorption in presence of each other. The data for the adsorption of phenols and heavy metal ions fit well into the Freundlich and Langmuir models. The Freundlich constants signify favourable adsorption of all the adsorbates. It also reflects a considerable degree of non-linearity between adsorbate concentration in solution and the extent of adsorption. Langmuir parameters have also been evaluated. The influence of the shape of isotherm on whether the adsorption is 'favourable' or 'unfavourable' has been considered using the method of Weber and Chakraborti(155) A comparison of the values of a dimensionless constant se paration factor indicate that the adsorption is favourable for all the adsorbates. -v- Thermodynamic parameters have also been derived. The Gibbs free energy values for all the adsorbates are negative thereby indicating the spontaneous nature of the process. The AH values are positive for phenols and negative for heavy metal ions. The entropy of adsorption is also positive in each case, thus indicating the affinity of the prepared adsorbent for the phenols and heavy metal ions under study. In order to have a quantitative understanding of the adsorption process and to determine the mechanism of the rate controlling step, kinetic studies have been performed. Preliminary investigations on the rate of adsorption of 4-nitrophenol, chromium, lead and mercury, on activated carbon indicate that adsorption occurs very rapidly. About 70 percent of the ultimate adsorption occurs within the first hour of contact. This initial rapid stage of adsorption then slows down as it approaches equilibrium. The rate of adsorption is found to be dependent on the amount and particle size of the adsorbent, temperature and initial adsorbate concentration. Diffusion theory has also been applied and it is found that both film and particle diffusion influence the process of diffusion. Column studies have been undertaken to design a fixed bed adsorber zone to establish itself, move down the length of the column of carbon and out of the bed; the time (t& ) required for the movement of the zone down its own length in the column The total time (t ) involved in the primary A -VIafter it has been established; the fractional capacity, f, of the carbon in the adsorption zone at breakpoint to continue to remove solute from solution; the length of the adsorption zone, 6 ,' and the percentage saturation at breakpoint have been evaluated for columns of 4-nitrophenol, chromium, mercury and lead. From the observations a direct relation ship is developed between the length of the adsorption zone ( 6 ) and percentage saturation at breakpoint. The desorption of 4-nitrophenol, chromium and lead has also been tried. 4-ni trophenol gets eluted by 5% NaOH whileCr 6+ and Pb 2+ are recovered with 3M NH 4Oil and 3M NaN03 respectively. 'In-situ' regeneration of the columns is possible by a treatment with 1. OM HNO, solution at a Clow rate of 0.5 ml min