THE SURFACE PROPERTIES OF NATURAL ILLITES AND THE ADSORPTION KINETICS OF Cu ONTO THEIR SURFACES

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1 THE SURFACE PROPERTIES OF NATURAL ILLITES AND THE ADSORPTION KINETICS OF Cu ONTO THEIR SURFACES Gonçalves, M. A. and Figueiras, J. Departamento de Geologia/CREMINER, Faculdade de Ciências da Universidade de Lisboa, Edifício C6, Piso 4, Campo Grande, Lisboa, Portugal. {mgoncalves, RESUMO O presente trabalho teve como motivação o estudo da dispersão superficial de metais pesados devida a lixiviados de aterros sanitários e sua distribuição em sedimentos, em particular atrvés da adsorção em minerais de argila. A titulação ácido-base de uma suspensão com 10g/L de ilite e uma força iónica de 0.1M KNO 3, permitiu determinar, por modelação inversa, o número de sítios superficias (5.05± mol/g) bem como as constantes de dissociação da superfície, pk a1 = 2.94±0.11 e pk a2 =3.98±0.14, de que resulta um ph pzc =3.46±0.13. As experiências de adsorção de Cu demostram que a capacidade de adsorção das superfícies da ilite aumenta com o ph, contudo, e de acordo com modelos de especiação química, a partir de ph=5 a fixação de Cu passa muito provavelmente pela formação de um precipitado na superfície. As experiências realizadas permitiram determinar a taxa de adsorção inicial de Cu e verificaram ainda que: 1) apenas cerca de um terço ou menos dos sítios com carga negativa formam ligações com o Cu em solução; 2) a introdução de ácidos monocarboxílicos não modifica a capacidade de adsorção de Cu, exceptuando a ph 6.5 de forma muito ligeira; 3) o aumento da força iónica decresce a capacidade de adsorção de Cu e aumenta a quantidade e a taxa de desadsorção de Cu. ABSTRACT This work has been motivated by the study of superficial heavy metal dispersion by landfill leachate-polluted waters and their distribution in stream sediments, especially adsorption onto clay minerals. Acid-base titration of a 10g/L suspension of illite in a 0.1M KNO 3 ionic strength medium allowed the determination of total surface site concentration (5.05± mol/g). The surface dissociation constants, pk a1 = 2.94±0.11 and pk a2 =3.98±0.14, thus resulting a ph pzc =3.46±0.13 were determined by inverse modeling of the chemical equilibrium system. The Cu adsorption experiments performed show that higher ph increases the illite surface adsorption capacity. Chemical equilibrium speciation models suggest that above ph 5 a surface Cu precipitate might form. The experiments also allowed the determination of the initial Cu adsorption rate and it also verified that: 1) only about a third or less of the negatively charged surface sites actually bond to the Cu in solution; 2) the addition of monocarboxylic acids did not change the Cu adsorption capacity, except for ph 6.5 where it decreased slightly; 3) the increase of ionic strength decreases the Cu adsorption capacity, and increases the total amount and rate of Cu desoprtion from the illite surface. Introduction Landfill leachates have an important fraction of low molecular weight organic acids in their early evolutionary stages, especially monocarboxylic acids (Christensen et al., 2001; Gonçalves et al., 2004; Schroth and Sposito, 1998), which can last for 2 to 10 years, depending on landfills. Leachate then evolve into the methanogenic stage, which is responsible for the development of higher molecular weight organic acids (humic- and fulvic-type acids), and of sparingly soluble metal phases (Pohland and Kim, 2000). Metals may be fixed by two main mechanisms: precipitation and adsorption. In the case of stream sediments, and in relation to Cu, adsorption is the most important mechanism and organic compounds play a fundamental role (Gonçalves et al., 2004). However, Zn and Pb form mainly precipitates, especially sulphides phases. Apart the overwhelming importance of organic compounds, phyllosilicates provide most of inorganic surface area available for adsorption, and in the context of the work previously made, it was verified that illite served a good reference material to study the properties of the inorganic surfaces available. Experimental Procedures In the experimental study we used the IMt-1 illite from the CMS repository of special clays. The pre-treatment of this illite included grinding of the sample and its subsequent washing and separation 513

2 in order to obtain a grain size fraction in the range of µm. Undesirable contaminant phases such as carbonates, iron oxides, and organic matter were removed following the procedures of Kunze and Dixon (1986). X-ray diffraction and SEM showed a relatively homogeneous 2M d non-expanding illite with a perfectly platy morphology. Quartz, in the order of 5% or less, and < 1% kaolinite were also detected. The illite fraction has an N 2 -BET surface area of ± 0.04 m 2 /g. The surface protonation constants as well as the reactive surface site concentration were determined by titration of a 10 g/l illite suspension in a 0.1 M KNO 3 solution and of an acid blank solution (Gonçalves, 2004). The titration was done in borosilicate glass vessels with a porous stopper and a continuous flow of pure N 2 gas to purge excess dissolved CO 2. The suspension was first titrated with 0.1M HNO 3 standardised against Tris-(hydroxymethyl) aminomethane (THAM) until a ph around 3 was achieved. The suspension was subsequently back-titrated with 0.1M KOH standardised against potassium hydrogen phtalate, up to a ph above 10. A chemical equilibrium model for surface speciation was fitted to the experimental results using FITEQL (Herbelin and Westall, 1999). Experiments to study the adsorption kinetics of Cu were done in a continuous stirred flowthrough reactor. Details of the experimental set-up are given in Gonçalves (2004). The experimental conditions were as follows: solution flow of 0.76 ml.min -1 ; stirring velocity of 850 rpm; T = 25ºC; 500 mg of illite. The reacting solutions had variable Cu concentrations ranging from 50 to 150 µm, ionic strengths of 10-4 M KClO 4 and 10-3 KNO 3, and ph values of 4.5, 5.5, and 6.5 adjusted with HNO 3. Two types of experiments were done: initial adsorption experiments, and break through curve (BTC) experiments, in which the reactor is initially filled with a blank solution and is then replaced by the Cu solution; once Cu saturation is achieved inside the reactor, the blank solution is again introduced in the system. The reacted solution was sampled regularly, and the ph was measured exsitu during each sampling interval. The analysis for Cu was done in a Philips Pye Unicam SP9 Atomic Absorption Spectrophotometer. Calibration was performed in each analytical session by the external standard method. The detection limit for Cu, under these conditions, was 100 ppb, but eventually we were able to pre-concentrate the more diluted samples and reach a 20 ppb detection limit with less than 20% uncertainty in the analysis. Acid-Base properties of illite surface A chemical speciation model for Al and Si in solution was fitted to the experimental titration curve of the blank solution and optimised with FITEQL (Herbelin and Westall, 1999). The input parameters to be optimised were taken from Du et al. (1997a). The optimised solubility constants were used as input in the illite suspension speciation model. Surface dissociation constants were optimised using the diffuse double layer model for the charged surface. Determination of the end points of the titration of both the blank solution and illite suspension with Gran Plots also allowed the estimation of the surface site concentration. The results from all these procedures are briefly summarised in Table I. Table I: Parameters optimised by FITEQL to the experimental points of the alkalimetric back-titration of the illite suspension; S stands for a surface site; * V Y corresponds to the FITEQL Goodness of Fit parameter. Characteristic Parameters of the Illite Suspension FITEQL Optimised Parameters Reaction Total Al 3+ (M) 6.24 ± Total Si (M) 3.46 ± Total SOH (mol/g) 5.05 ± pk a ± 0.11 SOH + H + + = SOH 2 pk a ± 0.14 SOH = SO + H + V Y * Form Titration Data Total SOH (mol/g) The IMt-1 illite has a ph zpc = 3.46 ± 0.13 according to the optimised surface dissociation constants and thus, beyond this ph value the surface is negatively charged (Fig. 1A). The general equation for Cu uptake at aqueous illite surfaces where there are n surface sites, m copper ions, and p protons is given by (Du et al., 1997b) 514

3 2+ (2m p) ( p n) n( SOH) + mcu + ( p n)h 2O ( SO) n Cu m (OH) + ph, (1) having an intrinsic adsorption constant int n (2m p) + [( SO) n Cu m (OH) ( p n) ][ H ] n 2+ m [( SOH) ] [ Cu ] p K, m, p = exp( (2m p) Fψ 0 / RT ). (2) In the equation, F is the Faraday constant, ψ 0 is the surface potential, R is the perfect gas constant, and T is the temperature. + A B 2+ [Cu ]=100 µ M Solid = g/l Figure 1: Surface speciation of the illite as a function of ph (A), and the equilibrium speciation with Cu (100 µm) the SOCu 2 (OH) 3 is a surface precipitate. Relevant equilibrium constants for Cu adsorption is given by Du et al. (1997b). The computed equilibrium speciation as a function of ph (Fig. 1B) shows that for the concentration range of the adsorption experiments, Cu adsorbs according to equation (1) with n, m, and p = 1. The other relevant reactions, (1,1,2) and (1,2,3) for the triplet (n, m, p), are not significant for these conditions. What is also important to outline, is the formation of a surface precipitate, which under equilibrium conditions, is likely to form at ph values greater than 5. As a matter of fact, Du et al. (1997b) present evidence for the formation of the Cu surface precipitate by means of FT-IR spectroscopy. Adsorption kinetics of Cu onto illite surfaces From the initial adsorption experiments the main results show the following: a) the system requires 1 to 2 hours before it reaches an initial steady state in which, irrespective of experiment initial conditions, Cu adsorption rates are maintained for 2 to 4 hours, and the ph in the reactor is between 6 and 6.5; b) after this initial stage, the adsorption edge develops and the system attains an output Cu concentration equal to its input concentration within 20 to 30 hours for input solution ph of 4.5 and 5.5; c) the higher the ph of the initial solution, the higher the adsorption capacity, and the longer the initial steady state; d) the adsorption edge curves for the experiments with ph 6.5 input solution and higher ionic strength develops earlier, but in these experiments the system did not reach saturation after 40 hours; e) hexanoic acid ( M) had no effect on Cu adsorption in the ph 4.5 and 5.5 experiments, but for the 6.5 experiment a slight decrease in Cu uptake was observed. The BTC experiments show that the ph effect on Cu adsorption (4.5 and 5.5) is greater at lower ionic strength, but on the whole, higher ionic strength decreases the Cu adsorption capacity onto the illite surface. Besides, higher ionic strength also causes the system to increase the initial Cu desorption form 23% to 37% at ph 4.5, and from 6% to 29% at ph 5.5, when the blank solution is reintroduced. Once this initial stage of rapid Cu desorption ends, the system slowly releases Cu, at an almost constant rate. Apparent rates of Cu adsorption, at 25ºC, ph range 6 6.5, and an excess adsorbent concentration (so that the fraction of unoccupied sites is 1) were computed form the initial steady state observed in some of the experiments. The simplified rate equation (where C Cu is the initial Cu concentration), with the associated uncertainty is Cu d ± - = 3.50 ± C (mol.m.min - ), 2 Cu r = (3) dt 515

4 Thus, Cu adsorption is a first order reaction, which is in agreement with what some studies have previously suggested. The Cu desorption rates computed from the BTC experiments are 1 to 2 orders of magnitude lower than the corresponding adsorption rates (for the same experimental conditions). The observed range of values is the effect of ionic strength increase leading to a corresponding increase in desorption rates by 65% (ph 4.5) and 57% (ph 5.5). It has been recently demonstrated that from flow-through reactor experimental data adsorption isotherms could be measured (Grolimund et al., 1995). For the experimental conditions used, the experimental points lie in the upper flat side of the curve of a Langmuir-type adsorption isotherm, which agrees with the observation that almost every experiment reached saturation. Total adsorbed Cu was computed from the experimental data points by numerical integration of the interpolated spline function with Lagrange end conditions. Using the maximum adsorbed Cu for each ph and the fraction of negatively charged sites according to the fitted surface speciation model, one gets an occupancy of 39% for ph 4.5, 22% for ph 5.5, and 24% for ph 6.5. Thus, only about a third or less of the negatively charged surface sites actually bind Cu from the solution. These results show some discrepancy with the equilibrium calculations in Fig. 1B, which is understandable because the experimental system is dynamical. The results are almost identical for ph 5.5 and 6.5, but are clearly different for ph 4.5. These results may actually express the transient status of the ph inside the reactor during the experiments, especially for a ph of 4.5, because even in these ones the ph initially increases to values above 6 during the period where Cu adsorption rates are the greatest. It is hypothesised that the system actually tends to restore equilibrium with solution, but the relaxation time to achieve this is much longer than the running time of the experiments. For the remaining experiments, this effect becomes less and less pronounced. However, it raises the question of whether there is effectively Cu precipitation in the system or not. This might be in the form of a surface precipitate, which is the thermodynamically favoured phase, or of a chloride phase, which requires Cl concentrations well above the ones used. These questions can only be answered using spectroscopic methods. Conclusions The adsorption of Cu follows a Langmuir-type adsorption isotherm, and is a first-order reaction. Monocarboxylic acids had little influence of the adsorption kinetics, in spite of being a major component in the leachate. Higher ionic strength solutions (as one would expect from polluted sources) decrease Cu adsorption capacity and increase Cu desorption kinetics. Formation of surface precipitates and the details of the adsorption mechanisms are, at present, still an open question. Acknowledgments This research work has been funded through project MODELWASTE POCTI/CTA/43390/2001 (FCT-MCES) References Christensen T. H., Kjeldsen P., Bjerg P. L., Jensen D. L., Christensen J. B., Baun A., Albrechtsen H.- J., Heron G., Biogeochemistry of leachate plumes. Appl. Geochem. 16, Du Q., Sun Z., Forsling W., Tang H., 1997a. Acid-base properties of aqueous illite surfaces. J. Colloid Interf. Sci. 187, Du Q., Sun Z., Forsling W., Tang H., 1997b. Adsorption of copper at aqueous illite surfaces. J. Colloid Interf. Sci. 187, Gonçalves M. A., Nogueira J. M. F., Figueiras J., Putnis C. V., Almeida C., Base-metals and organic content in stream sediments in the vicinity of a landfill. Appl. Geochem. 19, Gonçalves M. A. C., Heavy Metal Dispersion by Landfill Contaminated Waters and Fixation Mechanisms on Phyllosilicates: a Combined Field and Experimental Study. PhD Thesis, Universidade de Lisboa, 211 pp. Grolimund D., Borkovec M., Federer P., Sticher H., Measurement of sorption isotherms with flow-through reactors. Environ. Sci. Technol. 29, Herbelin A. L. and Westall J. C. (1999) FITEQL: A Computer Program for Determination of Chemical Equilibrium Constants from Experimental Data. Dept. Chemistry, OSU. 516

5 Kunze G. W. and Dixon J. B., Pretreatment for mineralogical analysis. In Methods of Soil Science Analysis. Part 1. Physical and Mineralogical Methods, Vol. 9 (ed. A. Klute), pp American Society of Agronomy - Soil Science Society of America. Pohland F. G. and Kim J. C., Microbially mediated attenuation potential of landfill bioreactor systems. Water Sci. Technol. 41, Schroth B. K. and Sposito G., Effect of landfill leachate organic acids on trace metal adsorption by kaolinite. Environ. Sci. Technol. 32,

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