Abstract
CHAPTER 1 INTRODUCTION
A large number of batch studies have been reported for heavy metal adsorption from mono-metal ion systems on different adsorbents over the past few decades (Ayoob & Gupta, 2008; Babel & Kurniawan, 2003; Febrianto et al., 2009; Foo & Hameed, 2009). However, industrial wastewaters rarely contain a single heavy metal ion. For example, waste effluent from mining, metallurgical, tannery, chemical and battery manufacturing contain Cu(II), Pb(II) and Cd(II) in varying concentrations (Li et al., 2011; Meena et al., 2005). Ni(II) and Cd(II) ions are frequently encountered in mine drainage, effluents from plating plants, paint and ink formulation units (Srivastava et al., 2006). Even groundwater may be contaminated with more than one metal ion such as As(III), Fe(II), Mn(II), Cu(II) and Pb(II) in varying proportions (Murugesan et al., 2006; Vilensky et al., 2002). Adsorption process is considered as one of the preferred methods for removal and recovery of heavy metals from industrial effluents and groundwater. In the recent past, a few efforts have been directed to understand kinetics and equilibrium of heavy metal adsorption from binary- and multi-metal ion systems which are critically reviewed in subsequent paragraphs.
The kinetics of metal biosorption from binary-metal ion system comprising of Cr(VI) and Fe(III) on C. vulgaris and R. arrhizus have been investigated with an aim to compare the effect of increase in concentration of Cr(VI) ion on the initial adsorption rate of Fe(III) or vice-versa (Sağ et al., 1998a). It reported decrease in initial adsorption rate of one metal ion with increase in concentration of the other and the combined biosorptive action of Cr(VI) and Fe(III) ions on C. vulgaris and R. arrhizus was found to be antagonistic. Similar studies were also carried out for biosorption of Cu(II) and Zn(II) on R. arrhizus (Sağ et al., 1998b).
However, these studies did not address the variation in solution pH during the process and consequently the metal removal mechanisms i.e. either by precipitation or adsorption or combination of both. Further these studies have failed to investigate simultaneous biosorption of metal ions from binary-metal ion system.
The kinetics of metal uptake was investigated from mono-metal ion systems of Cd(II) and Ni(II) onto bagasse fly ash (Srivastava et al., 2006). The solution pH was observed to increase from an initial pH of 6.0 to 8.4 for Cd(II) and 8.3 for Ni(II) within a short span of
precipitate at a pH value of > 7.7. The availability of Cd(II) ions and formation of Ni(II)- hydroxide precipitate with change in solution pH could only be assessed if the experiments were carried out using binary-metal ion system of Cd(II) and Ni(II), which was not attempted in this study (Srivastava et al., 2006). Further, simultaneous adsorption of both the metal ions from the binary-metal ion system might have taken place on the adsorbent. It indicates a need to carry out kinetic studies with binary-metal ion system to evaluate the overall metal removal under varying or a fixed pH conditions.
The importance of simultaneous adsorption of two or more heavy metal ions and presence of interactive effects between metal ions have led to an elaborate investigation to understand the competitive binding of Cu(II), Pb(II) and Cd(II) onto an iminodiacetic acid (IDA) chelating resin from mono- and binary-metal ion systems (Li et al., 2011). The kinetics were carried out using fixed initial concentrations of 0.5, 1.0 and 2.0 mmol/L for the target metal ion in presence of equi-molar concentration of interferential metal ion in the binary- metal ion systems. The metal uptake profiles from binary-metal ion systems indicated decreased metal uptake compared to respective mono-metal ion system with increase in interferential metal ion concentration indicating competition amongst metal ions for adsorption sites. However, total metal uptake capacities obtained from binary-metal ion systems investigated were quite higher compared to the same of mono-metal ion systems.
The difference in metal uptake capacities from mono- and binary-metal ion systems could be attributed to increased total metal ion concentration initially present in the solution in case of binary-metal ion systems, thereby increasing the driving force – concentration gradient, compared to mono-metal ion systems for the fixed number of adsorption sites available. The kinetics of metal uptake should have also been investigated by maintaining a fixed total initial metal ion concentration in the solution thereby keeping the driving force unchanged in mono- and binary-metal ion systems. Similarly multi-metal ion adsorption equilibrium studies were investigated wherein increased total initial metal ion concentration in multi-metal ion systems resulted in decreased uptake of metal ions compared to uptake from mono-metal ion systems and based on these results interactive effect of metal ions have been discussed (Baig et al., 2009; Sağ et al., 2001; Srivastava et al., 2006). Further, the results of most of the metal
reported on mass basis. As long as only mono-metal ion systems are being handled, this approach appears to be logical. However, when adsorption processes are applied for binary- and/or multi-metal ion systems where all the metals are going onto the surface of the adsorbent in different proportions simultaneously, then possibly expressing results on mass basis may lead to complicacy. Many of the authors working on the removal of metal ions from multi-metal ion systems have reported their results on mass basis considering only a particular metal ion while indicating the presence of other metal ions in the aqueous phase without quantifying its fate in the aqueous phase (Baig et al., 2009; Sağ et al., 2001;
Srivastava et al., 2006). This approach fails to indicate the overall metal-uptake capacity of adsorbents tested. Some of the authors have investigated the kinetics and/or equilibrium by considering a fixed initial concentration of individual metal ions both in the mono- as well as multi-metal ion systems and have attempted to compare the results. However, in this case, the concentration gradients might have been entirely different in mono- and multi-metal ion systems and therefore, the comparison of adsorption results might not be justified. Further in many adsorption studies, pH of the solution was not controlled during adsorption process which might have resulted in removal of metal ions by precipitation along with adsorption.
Although batch laboratory adsorption studies provide useful information on the application of adsorption to the removal of specific waste constituents, continuous column studies provide the most practical application of this process in water/wastewater treatment.
A few multi-metal column studies such as the biosorption of Cu(II), Pb(II), Zn(II) and Ni(II) from a mixed solution of the metals on polyurethane immobilised biomass (Zhang & Banks, 2006), removal of Pb(II), Cu(II), Zn(II) and Mn(II) from the leachate collected from the open dumping site on rice husk (Mohan & Sreelakshmi, 2008) and biosorption of Pb(II) and Cd(II) from aqueous solutions by protonated Sargassum glaucescens biomass (Naddafi et al., 2007) have aimed to study preferential adsorption of metal ions by comparing obtained breakthrough curves for metal ions from mono-metal ion system with the same obtained from mixture of metal ions. However, different experimental conditions in terms of total initial metal ions concentration was employed in mono- and multi-metal ions systems which eventually resulted in competition among metal ions for adsorption sites. Hence it is necessary to carry out mono- and multi-metal ion system column studies with fixed total initial metal ions concentration in solution. Further after exhaustion of column bed with one metal, its regeneration was carried out using different eluents. However literature is not
Hence present research work was aimed to investigate metal removal from mono-, binary- and ternary-metal ion systems with a fixed total initial metal ion concentration in the solution. The studies were carried out under uncontrolled and controlled pH conditions so as to compare metal removals under uncontrolled pH conditions and metal uptakes under controlled pH conditions. The metal ions concentrations were expressed on equivalent basis so as to take into account the differences in valence and atomic weights of selected metal ions as well as to facilitate algebraic summation of concentrations of metal ions which will eventually help in comparison of overall metal removal and overall metal uptake from mono-, binary- and ternary-metal ion systems. Batch studies for mono-, binary- and ternary-metal ion systems were carried out with an aim to investigate (a) the availability of metal ions in aqueous phase at different solution pH (b) effect of variation in contact time on metal ion concentration remaining in solution under uncontrolled and controlled pH conditions (c) kinetics of metal removal under uncontrolled pH conditions and metal uptake under controlled pH conditions and (d) equilibrium metal removal under uncontrolled pH conditions and equilibrium metal uptake under controlled pH conditions. Further, potential of previously loaded adsorbent with a metal ion or combination of binary-metal ion system for additional metal removal under uncontrolled pH conditions and additional metal uptake under controlled pH conditions for the same metal ion and/or to another metal ion or combination of binary-metal ion system were also investigated. The continuous mode column studies were also carried out for mono- and binary-metal ion systems by maintaining a fixed value of total initial metal ion concentrations in the solution under uncontrolled and controlled pH conditions. The continuous mode column studies in this work have been planned to investigate variation in pH of effluents coming out of the column beds, additional metal removal potential of exhausted/loaded bed if reloaded with another metal ion and migration of previously removed metal ion from the bed into the effluent when reloaded with another metal ion, if any.