Removal of lead ions from aqueous solution by retorted shale

The equilibrium and kinetic properties of Pb 2+ ion adsorption by retorted shale (RS) have been investigated during a series of batch adsorption experiments. The effects of pH, contact time and adsorbate initial concentration were evaluated. The pseudo-second-order model was used to predict the rate constants of adsorption system. Langmuir and Freundlich models were used to ﬁt the equilibrium data, which showed that Langmuir best-ﬁtted these data. Thermodynamic parameters of adsorption indicate spontaneous and endothermic nature of the process.


Introduction
The removal of toxic heavy-metal ions from industrial waste effluents has been widely studied in recent years.When they are released into the water, most of them are strongly retained and their adverse effects can last for long periods.Many of heavymetal ions are extremely toxic and threaten the living organisms by joining the food chain.Lead, chromium, cadmium, copper, zinc, and mercury are among the most frequently observed metal contaminants.
Heavy metals are generally removed from wastewaters by chemical precipitation, ion exchange, electrodeposition, membrane systems, and adsorption.In the last few decades, adsorption process has received much attention and represents, at present, a competitive and effective process for the removal of low concentrations of heavy metals [1].The literature reveals that a large number of cheap and abundantly materials have been studied as adsorbent.Some of these materials are peat [2], clay [3], diatomite [4], chitosan [5], siderite [6], zeolita [7], sugar beet pulp [8] spent grain [9], sago waste [10], and others.However, there has been no report on the application of retorted shale (RS) as adsorbent.
The present paper is an attempt to explore a possibility to utilize retorted shale to remove lead ions from waste water.This material is waste product obtained of pyrolysis (550 • C) of oil shale.Since RS holds several functional groups they would have high potential for heavy metals adsorption.In addition, the technological development for the beneficial use of this material is needed, once oil shale is abundant in 31 countries [11], and Brazil has the second largest reserve in the world.The kinetics and thermodynamic of adsorption was studied.The factors investigated included ion initial concentration, contact time and pH of solution.

Materials and methods
The retorted shale used in this study was obtained from São Mateus do Sul, State of Paraná, Southern Brazil.It was ground, sieved down to 75 m and used without any previous treatment.All the chemicals were of reagent grade purity.Deionized water (DI) was used to prepare all synthetic Pb 2+ solutions.
Physical and chemical analyses of the retorted shale were carried out by using conventional chemical methods [12] and available analytical techniques.The chemical composition of RS was determined by XRF analysis.The specific surface area of RS was determined by the BET method from the adsorption-desorption isotherm of nitrogen at its liquid temperature (77 K) (NOVA 2000 BET system).Particle size was obtained in a SILAS model 1064 analyzer.Shale density measure was carried out by picnometry technique.The experiments for adsorption kinetics were performed in batch reactors on a shaking incubator (TECNAL).

Adsorption experiments 2.2.1. Batch experiments
Batch adsorption experiments were carried out by shaking 1 g of adsorbent with 100 mL of Pb 2+ solution of the desired concentration at constant temperatures (30, 40 and 50 • C) and 260 rpm.Aliquots were taken at specific time intervals and then the adsorbent was removed by filtration.The concentration of Pb 2+ ions in the supernatant solution was measured by atomic absorption spectroscopy (Varian, Espectr AA-110 model).The effect of pH on adsorption was studied at 30 • C. The pH adjustments were done adding 1.0 M NaOH or HCl in the Pb 2+ solutions.
The amount of Pb 2+ adsorbed and the percent adsorption (%) were calculated as follows: where q t is the amount of Pb 2+ ion adsorbed (mg g −1 ) at different times (t), V is the volume of the solution (L), W is the weight of the adsorbent (g), C 0 and C t are the Pb 2+ concentration (mg L −1 ) at the initial and at a time t, respectively.

Characterization of retorted shale
The chemical constituents of the RS and its important physical parameters are presented in Table 1.In addition to the major  chemical components, the RS also contains MnO, V 2 O 5 , SrO, CuO, ZnO, ZrO 2 , NiO and sulfur.It is observed that retorted shale is mainly composed by acids, bases and amphoteric oxides, which assure the presence of active groups of mineral species and organic residues on the grain surface, suggesting good adsorption behavior.

Effect of initial pH
pH is one of the most important factors in the adsorption of metal ions in aqueous solutions, since it influences the solution chemistry of the heavy metals (i.e.hydrolysis, complexation, redox reactions, precipitation) also strongly influence the speciation and the sorption availability of the heavy metals.Fig. 1 shows the effect of the solution pH on the adsorption of Pb 2+ at 30 • C, for an initial concentration of 200 mg L −1 .The amount of Pb 2+ ion adsorbed is rather small for low pH values and increases as the pH increases, presenting a deeper increase between pH 2.5 and 4. Finally, a maximum is attained at around pH 5.5.Above pH 5.5 insoluble lead hydroxide starts precipitating from solution, making true sorption studies impossible.
Retorted shale contains aluminosilicates, a variety of hydrous oxide minerals and organic matter.These species contain surface hydroxyl groups that can undergo amphoteric dissociation reactions and react with metal ions.This kind of adsorption involves surface hydroxyl groups to form mono and binuclear inner sphere complexes and can be described as follows for a metal cation Me 2+ and a surface group S [13].

S-OH
In both cases the increase in the solution pH favors the incorporation of the ions to the adsorbent surface.

Adsorption kinetics
The experimental results for Pb 2+ adsorption on RS for various concentrations at 30 • C is shown in Fig. 2. As expected, the more concentrated the solution is, the adsorption.Although the initial adsorption is relatively fast, the equilibrium time required for a maximum Pb 2+ removal was 240 min.No further uptake was observed even after this time of contact.The plots of metal uptake as a function of time are single, smooth and continuous leading to saturation, suggesting the possible monolayer coverage of Pb 2+ ions on the surface of the adsorbent.
The kinetic of adsorption of Pb 2+ ions were modeled using the pseudo-first-order and pseudo-second-order equations.The first-order rate equation of Lagergren is one of the most widely used for the adsorption of solute from a liquid solution [14].It may be represented as follows: where k 1 (min −1 ) is the equilibrium rate constant of the pseudofirst-order adsorption, and q e and q t (mg g −1 ) are the amount of metal ions adsorbed at equilibrium and at any time t.Eq. ( 3) after integration and by applying the boundary conditions, q t = 0 at t = 0, and q t = q t at t = t, gives ln(q e − q t ) = ln q e − k 1 , ( The pseudo-second-order rate equation developed by Ref. [7] is expressed as: where k 2 (g mg −1 min −1 ) is the rate constant of the pseudosecond-order adsorption process, and q e and q t (mg g −1 ) are the amount of metal ions adsorbed at equilibrium and at any time t.Integrating Eq. ( 5) and taking into account that the initial adsorption rate (V 0 ) is, Eq. ( 5) leads to: Fig. 3a shows the pseudo-second-order plot for Pb 2+ adsorption at in the initial concentration of 200 mg L −1 at three temperatures and Fig. 3b shows the plot at different initial concentrations and 30 • C. The constants calculated for 200 and 400 mg L −1 concentrations at 30, 40 and 50 • C using both rate equations are shown in Table 2.The regression coefficients for the linear plots from pseudo-second-order equation were greater than those obtained for the pseudo-first equation indicating that this model was not applicable for all the results.Therefore, no further consideration was given to this model.The correlation coefficients for the pseudo-second-order are superior to 0.999 in all the systems.This suggests that the sorption system is not a first-order process and that a pseudo-second-order model can be considered.This latter model is based on the assumption that the rate limiting step may be chemisorption, involving valence forces through sharing or exchange of electrons between adsorbent and adsorbate.
The equilibrium rate constant of the pseudo-second-order adsorption, k 2 , increase with an increase in temperature for the initial Pb 2+ concentrations indicating that the adsorption process becomes slower for higher temperatures.This result may be interpreted as an indication of some specific interaction between the solid adsorbent and the lead ions.If the adsorption rate were where, k is the rate constant at temperature of T (K), A the frequency factor, R the universal gas constant (8.314J mol −1 K −1 ) and E a (kJ mol −1 ) is the activation energy for the adsorption process.The activation energy was obtained from slope of the plot of ln k values versus 1/T (figure not shown) and was found to be 30.89kJ mol −1 .This value suggests that the adsorption process involves chemical forces.

Adsorption isotherms
Adsorption isotherms are important to describe how solutes interact with adsorbents and to design adsorption systems for practical or technological use.Two equilibrium models were used: the Langmuir and Freundlich isotherms.The Langmuir model is perhaps the best known isotherms for describing sorption from a liquid solution.The model assumes uniform energies of adsorption onto the surface and no interaction between adsorbed molecules.The model also assumes that adsorption is limited to complete surface coverage by a monomolecular layer and can be representing by following equation [15]: The equation above can be rearranged to its form linear: where C e is the solution concentration at equilibrium (mg L −1 ), q e the amount adsorbed at equilibrium (mg g −1 ), K L the Langmuir constant (L mg −1 ) which can be considered as a measure of the adsorption energy and b is the maximum adsorption capacity (mg g −1 ) corresponding to complete monolayer coverage.A plot of C e /q e versus C e over the entire concentration range produces a straight line, which is an indication of the applicability of the Langmuir isotherm for the system under consideration.
The Freundlich model assumes that different sites with several adsorption energies are involved in the adsorption process.The Freundlich isotherm is shown as the following equation: The linear form of the equation can be written as: where K F and n are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively [16].
The intercept and the slope of the linear plot of ln q e versus ln C e at given experimental conditions provide the values of K F and 1/n, respectively.The linearized Langmuir and Freundlich adsorption isotherms obtained at 30, 40 and 50 • C are given in Figs. 4 and 5, respectively.The adsorption constants evaluated from the isotherms and their correlation coefficients are shown in Table 3.In general, the Langmuir model fitted the results slightly better than the Freundlich model with all R 2 values greater than 0.98.This suggests that the adsorption of Pb 2+ ions by RS is monolayer-type and agrees with the observation that the metal ion adsorption from an aqueous solution usually forms a layer on the adsorbent surface [17].To a lesser extent, the equilibrium   data was also well described with the Freundlich model, probably due to the real heterogeneous nature of the surface sites involved in the metal uptake.The fact that both monolayer and heterogeneous surface conditions exist under the experimental condition used, implies that the adsorption of Pb 2+ ions on the RS is thus complex, involving more than one mechanism.It is also observed that the values of Langmuir constants, b and K L , increased with increasing the temperature, showing that the adsorption capacity and intensity of adsorption are enhanced at higher temperatures and suggesting that there is a chemical interaction between adsorbent and adsorbate.As seen from Table 3, the monolayer maximum adsorption capacity (b) was 38.55 mg g −1 at optimum pH (5.5) and temperature (50 • C).
A comparison of the maximum capacity, b, of RS with other adsorbents reported in literature is given in Table 4. Differences  of metal uptake are due to the properties of each adsorbent such as structure, functional groups and surface areas.The adsorption capacity of RS was relatively high when compared with other adsorbents of inorganic origin such as diatomite, clay and siderite.However, the adsorption capacity is much lower than some sorbents of organic origin such as peat and chitosan.The capacity of chitosan is almost 21 times higher than RS, however, due the chitosan be nonporous and soluble in acidic solution, it becomes necessary to be modified to improve its sorption capacity.

Thermodynamics studies
Thermodynamic parameters such as Gibbs free energy ( G o ) enthalpy ( H o ) and entropy ( S o ) for the adsorption process can be calculated from the van' Hoff equation [18].
where R is the ideal gas constant, T temperature (K) and K L is Langmuir's constant.The slope and intercept of the plots of ln K L versus 1/T were used to determine H o and S o . ln As seen from Table 5, the negative values of G o indicate the spontaneous nature of the adsorption process.The Gibbs free energy indicates the degree of driving force of the adsorption process, where more negative values reflect a more energetically favorable adsorption process [19].The increase in free energy change with increasing the temperature shows an increase in the feasibility of adsorption at higher temperatures.The positive value of H o indicates the endothermic nature of adsorption and suggests the possibility of weak bonding between adsorbate and adsorbent.The positive value of S o suggests some structural changes in adsorbent and adsorbate.

Conclusions
The present investigation shows that the retorted shale can be used as an adsorbent for the effective removal of Pb 2+ from aqueous solution.The Langmuir equation describes very well with the equilibrium isotherm for the three temperatures and the entire concentration range we studied.However, to fit the isotherm data by the Freundlich equation also gives good correlation coefficient.The adsorption capacity increases with an increase in temperature and positive enthalpy value of process shows that the adsorption process is endothermic.The kinetics of adsorption can be described by a model of a pseudo-secondorder because of the strong correlation of the experimental results obtained using the Ho and McKay's linearization model.Thermodynamic parameters indicate that the Pb 2+ adsorption on RS is endothermic and kinetic studies suggest that adsorption involved some activated or chemisorption process.

Fig. 2 .
Fig. 2. Effect of contact time and initial concentration on the Pb 2+ adsorption by RS at 30 • C.

Fig. 3 .
Fig. 3. Pseudo-second-order plot of Pb 2+ adsorption kinetics by RS: (a) at different temperatures and (b) at different concentrations and 30 • C temperature.

Table 2
Kinetic parameters for Pb 2+ adsorption by RS at different temperatures

Table 3
Correlation coefficients and parameters for Langmuir and Freundlich isotherms of Pb 2+ adsorption by RS L (L mg −1 ) b (mg g −1 ) R 2 K F (mg g −1 ) 1/n

Table 4
Adsorption maximum capacities of this work and the literature