Adsorption Property of Pb ( II ) by the Laterite-Bentonite Mixture Used as Waste Landfill

Key Laboratory of Geotechnical & Underground Engineering of Ministry of Education, Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China School of Civil Engineering and Architecture, Changsha University of Science and Technology, Changsha 410114, China Laboratoire Navier/CERMES, Ecole des Ponts-ParisTech, 6-8 Av. Blaise Pascal, Cité Descartes, 77455 Marne-la-Vallée, France


Introduction
With the economic and social development of many countries in the past few decades, industrialization and urbanization grew rapidly along with the great increase in both industrial and municipal solid waste production.In China, the overall volume of municipal solid waste (MSW) increases at a rate of 5% to 8% per year, making the effective disposal of pollution becoming an urgent environmental problem.Generally, there are three main methods for MSW disposal: composting, incineration, and landfilling.By studying the exploiting and capital cost and the treatment efficiency, the sanitary landfill method, which allows the pollutant to decompose and becomes stabilized under control, is widely accepted and used in terms of its economic advantages [1].However, the leachate, a poisonous and harmful liquid, forms in the process due to the underground water and precipitation, and it contains many heavy metal ions such as Zn(II), Pb(II), Cr(III), Cd(II), Ni(II), and Cu(II) [2].If these toxic metals move out of the landfills by advection and diffusion, the surrounding environment may be contaminated and the health of living plants and animals will be jeopardized [3,4].An effective way to restrict the migration of toxic ions is the usage of a liner to absorb these metal ions and separate the environment from MSW [5].
To achieve this goal, the high adsorption of the material used as liners is a crucial prerequisite [6,7].However, there are few natural soils and rocks at the landfill that could meet this requirement.For example, laterite, a kind of soil which is distributed wildly in tropical and subtropical regions of South China, is an expected source of liner material whose mechanical property is good.However, the tectonic fissure and low adsorption capacity bring some challenges for its safe application as the landfill liner for MSW [8][9][10].erefore, a lot of additives have been studied to improve the property of natural materials such as lime, cement, fly ash, clay soils, and shale to improve their adsorption [6,8,[11][12][13].Among them, using bentonite as additive or the main material which has high cation exchange capacity (CEC) and specific surface area (SSA) can improve the adsorption property of the liner significantly and reduce its hydraulic conductivity effectively [14,15].e literatures showed that the adsorption property has been influenced by contact time, solid-liquid ratio, pH, ionic strength, and temperature [16][17][18].However, most of the literatures concentrate on the property of pure bentonite and natural clay [19][20][21][22][23]. Scarce literatures discuss the property, especially the adsorption capacity of lateritebentonite mixture other than its hydraulic conductivity [24,25] and shear strength [26].
In China, the laterite has been considered as the ideal material due to its small settlement, good overall performance, and low costs in many places of South China.Besides, Gaomiaozi (GMZ) bentonite from north of China is a promising additive due to its high CEC, large SSA, and low permeability.To guarantee the safe and long-term application of laterite-GMZ bentonite mixture used as MSW landfill liner, a batch test has been undertaken and effects of bentonite content, contact time, solid-liquid ratio, ionic strength, pH of solution, and temperature on the sorption of Pb(II) are discussed.

Materials and Apparatus.
Laterite used in the present work was taken from Chenzhou City, Hunan province, China, and has been air-dried and sieved (0.5 mm sieve) for use.Some geotechnical and chemical property of this red powder are listed in Table 1 after standardized test (JTGE40-2007) [27] and XRF (X-ray fluorescence spectrometer) test.As can be seen, the laterite is composed of several minerals such as 80.2% kaolinite, 12.5% quartz, and 6.4% monticellite.
e GMZ bentonite was sampled from Gaomiaozi township in Northern China, and it had also been air-dried and sieved (0.5 mm sieve) for use.e basic properties of this light-grey powder are listed in Table 2 [28,29].As shown in the table, its main mineral component is montmorillonite (75.4%).
Pb(NO 3 ) 2 , a white crystal, used in this experiment was bought from the Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China), and its molecular weight is 331.21.e purity of this solid is more than 99.0%, and it is a kind of dangerous chemical which can be used as strong oxidant.
In the test procedures, three apparatus were used, including a centrifuge, constant temperature incubator, and inductively coupled plasma (ICP).e type of the centrifuge is TD3(800B) whose maximum relative centrifugal force (RCF) is 1980 × g.
e ICP used in this study was produced by Agilent Technologies (California, America) whose type is PE8300.

Adsorption Test.
Adsorption experiments were conducted using the batch test according to the ASTM (1993) standard [30] under two temperatures of 298 K and 323 K.
e laterite and bentonite used in this test were mixed at the dry mass ratios of 1 : 0, 9 : 1, 8 : 2, and 0 : 1 and have been sieved with 0.5 mm mesh.Pb(II) solution (100 mg•L −1 ) was prepared by dissolving Pb(NO 3 ) 2 solid in distilled water under the specific mass ratio.e experiment procedures are as follows.
Soil powder and Pb(II) solution were mixed at different solid-liquid ratios (s/w) in the Erlenmeyer flask which contains 200 mL solution.e Erlenmeyer flask was sealed on the top and placed in a constant temperature shaking incubator at 180 rpm for 8 hours.At a given time, the Erlenmeyer flask was pulled out and placed in a centrifuge for 10 min at 9400 rpm for solid-liquid separation.e supernatant solution was sampled by using a dropper and dripped into the testing tube of ICP.
en the ion concentration of Pb(II) of this solution was obtained, which is termed equilibrium concentrations (C e ).To study the influence of contact time on the sorption, the concentration of Pb(II) in the supernatant is tested at regular intervals, from 5 min to 480 min.e solid-liquid ratio (s/w) was set as 0.5 to 200 g•L −1 for the investigation of its influence on soil adsorption property.Considering the different effect of pH and ion strength of the solution, NaCl solid was added into Pb(II) solution with concentrations of 0 mol•L −1 , 0.01 mol•L −1 , 0.1 mol•L −1 , and 1 mol•L −1 and the pH of this solution was controlled from 2 to 11 in the experiment.
ere were 29 experiment conditions in total, and they are all listed in Table 3.

Influence of Contact Time and Adsorption Kinetics.
To study the influence of contact time of Pb(II) adsorption by laterite and its mixture with bentonite, the concentration of Pb(II) was measured at different contact times between soil particles and solution from 5 to 480 minutes.e experiment results are plotted in Figure 1.As can be drawn from the figures, the adsorption rate of Pb(II) increases with the increase in contact time and the equilibrium time of four different soils decreases from 30 minutes to 5 minutes apparently.erefore, the equilibrium contact time for other studies in this research is determined at 30 minutes.In Figure 1, the adsorption rate (%) of Pb(II) and the adsorption capacity of soil per unit mass (q e ) are expressed in following equation: where C i and C e (mg•L −1 ) are the initial Pb(II) concentration and equilibrium Pb(II) concentration in the solution, respectively; v (L) is the total volume of the solution; and m (g) is the mass of adsorbent (i.e., soil particles).
Based on the experiment data, the adsorption rate of Pb(II) within 5 minutes are 10.55%, 35.11%, 55.32%, and 99.17% when bentonite content equals to 0%, 10%, 20%, to 100%, respectively.e ultimate adsorption rate of this kind heavy metal ion is 12.83%, 38.06%, 57.80%, and 100% when the bentonite content varies in the mixture.Obviously, the equilibrium adsorption rate of pure bentonite is about 8 times to pure laterite, and this can be explained by the much higher adsorption capacity of bentonite than that of laterite [31].
To analyze the influence of time on adsorption, three adsorption kinetic models are proposed to delineate the adsorption process, which are pseudo-first-order kinetics equation, pseudo-second-order kinetics equation, and intraparticle diffusion kinetics equation (i.e., W-M kinetics equation), respectively [32].e pseudo-first-order kinetics equation and pseudosecond-order kinetics equation are based on the assumptions that the reaction rate is in direct proportion to the first and second power of reactant concentration.e equations can be expressed as follows: ln q e − q t  � ln q e − k 1 t, (2) where q t (mg•g −1 ) is the adsorbed quantity of Pb(II) on soil particles at time t (min) and q e (mg•g −1 ) is the equilibrium adsorption capacity.k 1 and k 2 (g•mg −1 •min −1 ) are the pseudo-first-order and pseudo-second-order rate constants of adsorption, respectively.e intraparticle diffusion kinetics equation was proposed by Weber and Morris based on three assumptions: (1) the resistance of film only works at the initial stage of  Advances in Civil Engineering adsorption in a short time or it is neglected; (2) the direction of diffusion is random and the concentration of adsorbate is constant; and (3) the intraparticle diffusion parameter is a constant.
where k i (g•mg −1 •min −1 ) is the intraparticle diffusion parameter and C (mg•g −1 ) is a constant.
Based on the three kinetics model, the fitting curves of the Pb(II) adsorption kinetics are plotted in Figure 2. e value of parameters in equations ( 2)-( 4) can be calculated and are listed in Table 4.As can be drawn from the table, the values of correlation coefficient (i.e., R 2 ) for pseudo-firstorder and pseudo-second-order kinetics models are greater than 0.99, indicating a good fitting effect especially for the pseudo-second-order kinetics model.Besides, similar results with the use of the pseudo-second-order kinetics model can be found in the literatures using bentonite [17,33,34] or laterite [21] as adsorbent.erefore, the proposed pseudosecond-order kinetics model can delineate this process very well.However, the correlation coefficients of intraparticle diffusion kinetics equation are far from 1, suggesting an improper use of this model.

Influence of s/w.
To study the dependence of Pb(II) adsorption on solid to solution ratio, varied s/w (0.5-200 g•L −1 ) adsorption experiments are conducted and the results are plotted in Figure 3.It is clear that the adsorption rate of Pb(II) ions increases with the increase of s/w.Some researchers attribute this phenomenon to the greater cation availability of exchangeable sites with the increase of s/w [20,33,35].When this value equals to 100 g•L −1 , 40 g•L −1 , 20 g•L −1 , and 5 g•L −1 for four laterite-bentonite mixtures whose mass content of bentonite is 0%, 10%, 20%, and 100%, respectively, the ultimate ion adsorption rate remains constant even when the solid mass into the solution is increased.Adding the soil particles into the solution does provide more adsorption sites, but the total quantities of Pb(II) cannot be adsorbed completely.
is is ascribed to the decreasing concentration gradient between solution and adsorbent surface when a majority of Pb(II) ions were absorbed by the soil particles [36].Besides, the equilibrium capacity of soil per unite mass decreases with the increase of s/w.Similar results have been found for Cd(II) on the GMZ bentonite [16] and lead on the treated laterite [37], and this can be credited to the competition of soil particles.
On the contrary, increasing content of bentonite in the soil mixture brings a higher equilibrium adsorption capacity and ultimate adsorption rate of Pb(II).ese results also indicate the higher cation exchange capacity of bentonite than that of laterite, leading to important influence on adsorption characteristic by the mass content of bentonite.Parallel results have been found in copper adsorption by sand-bentonite as liners [38] and Pb(II) adsorption by mixture of kaolinite-sago waste and bentonite-sago waste [39].

Influence of pH.
Because the existence of Pb(II) is highly dependent on the pH value, the batch test was carried out under different pH values [2][3][4][5][6][7][8][9][10][11], and the experiment results are shown in Figure 4. Clearly, the adsorption rate of Pb(II) increases significantly with the increase of the pH value, and this trend becomes evident when bentonite content decreases.When the pH value of the solution equals to 2, the adsorption rate of Pb(II) is 0%, 4.49%, 14.08%, and 64%, respectively, for the four soil mixtures and this value aggrandize approximately to 100% when the pH of the solution is 11. e increase in the speed of ion adsorption rate is sharp when pH varies from 2 to 8, and it reduces when the pH value exceeds 8. e extent of total increase in adsorption is about 36% to 100% which indicates that pH plays a paramount role in the adsorption of Pb(II).
e reason of the aforementioned experiment rules is ascribed to the change of hydrolysis degree of Pb(II) which depends highly on the pH of solution [45].As is shown in equation ( 5) and Figure 5, the majority of Pb(II) exists as free Pb 2+ when pH < 7.5 and K is the hydrolysis equilibrium constant of Pb(II): In this situation, the high concentration of H + will compete for the adsorption sites on soil particles.erefore, the adsorption rate of Pb(II) is scarce in this condition.With the rise of pH [7.5-9.5],more Pb(OH) + forms and a small part of Pb(II) has been hydrolyzed into Pb(OH) 2 which will enhance the adsorption of Pb(II).With the further increase of pH [9.5-11], most of Pb(II) separates out of the solution in the form of the Pb(OH) 2 sediment which has a strong influence on the equilibrium adsorption rate of this kind of heavy metal ion.
Besides, the surface property of soil particles will change with the increase of pH, such as surface charges of laterite and bentonite particles [37].When the pH value of the solution is low, the high concentration of H + leads to a positively charged surface of soil particles.e electrostatic repulsion between the surface charge and Pb(II) ions will result in a low adsorption rate of Pb(II).However, there is a specific pH value when the surface charge of soil particles equals to zero, which is named zero point charge (pH zpc ).When pH > pH zpc , the particle surface becomes negatively charged, leading to a high uptake of Pb(II) because of the electrostatic attraction between cations and anions.

Influence of Ion Strength.
To find out the effect of ion strength [Na + ] on Pb(II) adsorption, the NaCl solid was added into Pb(NO 3 ) 2 to obtain a solution whose sodium concentration are 0 mol•L −1 , 0.01 mol•L −1 , 0.1 mol•L −1 , and 1.0 mol•L −1 , respectively.e experiment result is presented in Figure 6.It is obvious that the increase of ion strength plays a crucial role on the adsorption of Pb(II).
e adsorption decreases sharply with the increase of NaCl concentration.For the four different soil mixtures, the 4 Advances in Civil Engineering adsorption rate of Pb(II) is 14.85%, 37.21%, 55.78% and 99.99%, respectively, when there is no Na + .However, the uptake extent of Pb(II) decreases to 4.63%, 4.94%, 9.51%, and 12.63% when the concentration of NaCl increases to 1.0 mol•L −1 , and this decline is fast within certain range [0-0.
. is phenomenon is more apparent when the mass content of bentonite is high which indicates that sodium ion has a greater in uence on bentonite than laterite.Similar results have been reported in the literatures, and they attribute the reason to a competitive adsorption or the cation exchange between the added cation and adsorbate [16].When Na + is at a low concentration [0-0.1 mol•L −1 ], its q t = 0.402 × t 0.5 + 14.560, R 2 = 0.231 q t = 0.247 × t 0.5 + 8.101, R 2 = 0.231 q t = 0.169 × t 0.5 + 5.179, R 2 = 0.231 q t = 0.066 × t 0.5 + 1.599, R 2 = 0.231 0  greater hydration power leads to a cation replacement on soil particles.e active site on laterite and bentonite particles will be occupied by Na + .When the adsorption of Na + saturates gradually with the increased concentration of NaCl, the decrease of Pb(II) adsorption rate slows down as shown in Figure 6 and other literatures [16].Mg 2+ and Ca 2+ have similar comparative e ect on Pb(II) adsorption and the e ect on these three cations can be arranged in the following order: Mg 2+ > Na + > Ca 2+ [37].
Other than cations, acid radicals also have an in uence on the adsorption rate of Pb(II).e radius of Cl − is smaller than NO 3 − , leading to a stronger complexation between Cl − and Pb 2+ .erefore, the concentration of Pb(II) is lower when adding NaCl into the solution than NaNO 3 .Besides, SO 4  2+ in the solution will also reduce the uptake rate of Pb(II) due to the chelate complex compound formed between SO 4 2+ and Pb 2+ .

In uence of Temperature and Isothermal Adsorption.
It is known that the environment temperature is a crucial factor to in uence the physicochemical behavior of metal ions adsorbed by materials.erefore, the batch test of Pb(II) adsorption is carried out at 25 °C (298 K) and 50 °C (323 K), and the experimental results are plotted in Figure 7. Clearly, the uptake of Pb(II) is higher when the temperature is higher, suggesting that the adsorption reaction of Pb(II) is an endothermic process.When the temperature rises from  )  6 Advances in Civil Engineering 289 K to 323 K and the equilibrium Pb(II) concentration (C e ) varies from 0 mg•L −1 to 1240 mg•L −1 , the increase of adsorption capacity of soil per unit mass (q e ) changes from 3 mg•g −1 to as high as 56.447 mg•g −1 for different lateritebentonite mixtures.is in uence e ect is stronger for soils with more bentonite.
To simulate the adsorption isotherm of the Pb(II) adsorption on surface of soil particles, the Langmuir isothermal model and Freundlich isothermal model are adopted in this study and previous works.ere are three assumptions of the Langmuir isothermal model: (1) adsorbate molecules are absorbed on the particle surface as a monolayer; (2) all the adsorption sites are the same; (3) there is no interaction between adsorbate molecules.e Langmuir isothermal model can be expressed as follows: where q max (mg•g −1 ) is the maximum adsorption capacity per unit mass, b L (L•mg −1 ) is the Langmuir adsorption constant which relates to the change of enthalpy of reaction process, and these can be calculated through the linear relationship between C e /q e and C e .
In contrast, the Freundlich isothermal model is an empirical formula for an inhomogeneous system as follows: where q e is already de ned in equation ( 6) and and n are the Freundlich isotherm parameters.e experiment data of the isotherm adsorption are tted using the Langmuir and Freundlich models, and the results are plotted in Figures 8 and 9.As can be drawn from the curves, the values of correlation coe cient (R 2 ) for soils using the Langmuir model are almost more than 0.99.But this value of the Freundlich tted curves has a great distance with 1. erefore, the adsorption behavior of Pb(II) can be accurately described by the Langmuir isotherm model.erefore, it can be conjectured that a majority of adsorbate molecules are absorbed on the particle surface as a monolayer which is consistent with the assumptions of the Langmuir isothermal model.e tted values of parameters in the Langmuir and Freundlich isotherm models are listed in Table 5.As can be drawn, with the rise of temperature, the maximum adsorption capacity per unit mass (q max ) and the value of K F also have an apparent increment.is e ect can be explained by the endothermic process of Pb(II) adsorption.In addition, K F in the Freundlich isotherm model is a parameter which represents the degree of adsorption.Its value increases obviously with the increase of ω b , indicating a greater in uence of bentonite on Pb(II) adsorption than that of laterite.Besides, the increase of q max in the Langmuir model also veri es this in uence.To predict the maximum adsorption of the laterite-bentonite mixture, a simple equation was proposed as follows: where ω b (%) is the bentonite content, q b (mg•g −1 ) and q l (mg•g −1 ) are the maximum adsorption of Pb(II) by pure laterite and bentonite, respectively, and q m is the maximum value of Pb(II) adsorption by the laterite-bentonite mixture.
When applying equation ( 8) at two di erent temperatures, the prediction equation above can be plotted in Figure 10.As is shown in the gure, the predicted value of maximum adsorption by mixture is almost equal to the experiment results, indicating the practical application of this prediction equation.

Conclusion
In this study, the adsorption of Pb(II) by laterite-bentonite mixture was investigated using the batch test under di erent conditions, including bentonite content (0-100%), contact time (5-480 min), solid to solution ratio (0.5-200 g•L −1 ), pH (2)(3)(4)(5)(6)(7)(8)(9)(10)(11), ion strength (0-1 mol•L −1 ), and temperature (328 K, 323 K).To evaluate the in uence of those conditions, three adsorption kinetic models and two adsorptive isotherm models are used to analyze the experiment data and the feasibility of di erent models in this research.Several important conclusions are drawn as follows: (1) With the increase of contact time and bentonite content, the uptake of Pb(II) will increase and reach an equilibrium concentration in the end.e higher the content of bentonite in the mixture is, the shorter the equilibrium time will be.In general, the adsorption process can reach the equilibrium within 30 minutes.e experiment data can be tted best  Advances in Civil Engineering with the pseudo-second-order kinetics equation among pseudo-rst-order kinetics, second-order kinetics, and intraparticle di usion kinetics equation.e Pb(II) adsorption rate of pure bentonite is about 8 times than laterite because of the much higher adsorption capacity of bentonite than laterite.(2) When increasing the s/w, the adsorption rate of Pb(II) ions will increase apparently because of the more adsorption sites in the Pb(NO 3 ) 2 solution.However, the equilibrium capacity of laterite-bentonite mixture will decrease because of the decreasing concentration gradient between solution and adsorbent surface.

Figure 5 :
Figure 5: Distribution of Pb(II) existence form as a function of pH.

Figure 3 :
Figure 3: In uence of s/w on (a) equilibrium capacity of Pb(II) and (b) Pb(II) adsorption rate (pH 5.5; C i 100 mg•L −1 ; T 298 K; t 30 min), ω b is the mass content of bentonite in the soil mixture.

Figure 10 :
Figure 10: Comparison between the predicted values and experimental results of maximum Pb(II) adsorption by laterite-bentonite mixture.

Table 1 :
Some geotechnical and chemical properties of laterite.

Table 3 :
Experiment conditions of Pb(II) adsorption property of soil.

Table 4 :
Values of pseudo-second-order kinetics parameters.

Table 5 :
Parameters of the Langmuir and Freundlich isotherm models.