Cu (II) Chemisorption on Calcined Substrates made with an Oxidic Refractory Variable Charges Lithological Material

Quimioadsorción de Cu (II) sobre un Sustrato Calcinado preparado con un Material Litológico Refractario de Carga Variable

Transitional metal adsorption in variable charge soils have been well studied by several authors [1, 2] but has not been treated for the case of calcined substrates prepared with oxidic refractory lithological materials. In both cases the Fe and Al, as well as Mn and Ti amphoteric oxides are the most important source of variable charge; these amphoteric surfaces can be either protonated or deprotonated by acid or alkaline treatment to create positive or negative charges on the oxides surfaces, leading to cation and anion adsorption reactions respectively, according to equation (1) [3].

The literature also suggest a mechanism for which the adsorption of transitional metals on these kind of surfaces, through the formation of a covalent bond between the metal ion and the oxidic surface, called chemisorption, according to equation (2) where M could be any transitional metal.

Such kind of reaction modifies the surface charge through more positive values, which allows anion adsorption, and produces acidification through the formation of H3O+ ion. This process is defined as specific adsorption or chemisorption, which has the tendency to be irreversibility. In previous publications [4, 5] copper adsorptions on calcined substrates prepared with some of these refractory lithologic materials which have surface variable charges were described. This physicochemical characteristic is due to the presence of amphoteric oxides in the material, such as Fe, Al, Mn and Ti, previously described in the literature [6, 7]. As a consequence of these particular properties, these lithological materials are versatile for preparing calcined adsorbing substrates and their applications in water treatment. Furthermore, due to their capacity for anion/cation exchange, heavy metals, oxyanions and organic matter are removed by adsorption processes. In previous publications the application to water softening [8], cation adsorption reactions [4], anion adsorption reactions [9, 10] and water treatment [11] have been described. The objective of this paper is to complement the information presented by the previous articles with new findings which support the hypothesis of the chemisorption of cupper ions on the oxidic surface of these calcined substrates, in order to continue working on this project. Moreover, according to the theoretic model described abode, H3O + ion must be one of the reaction products, producing acidification in the solution. Therefore, by following up the pH evolution during the adsorption reaction it should show this acidification process.

Geografic localization and characterization of thelithologic material have been described in the literature[7]. Being an arid zone, the soils are classified as aridisols[12], presenting serious limitations for agronomical uses.However, some of these lithological materials are used bypotters for making kitchen hardware and constructionsmaterials like bricks and crockery using thermal treatmentdue to its refractory properties. Calcined subtrates wereprepare according to the procedure described in theliterature [4 - 6], so by using the granulometric fractionbetween 425 – 250 mm for the determination of thezero charge point, the pH and the electrical conductivitymeasurements. The specific surface was measured bythe BET technique using isothermal N2 adsorption

The procedure for the deprotonation reaction of the calcined substrate (substrate activation) is also described in the same literature were the substrate is chemically treated with a NaOH 0.1 M solution during 12 h and finally theexcess of alkali is wash out with distillate water untilit reaches pH 7, and later it dries in a furnace at 120oC.The determination of Point of Zero Charge (PZC) of rawthe material, (RM), calcined non-activated (NAS) andactivated substrates (AS) was performed according tothe method described by the literature [13, 14]. The pHwas measured at different ionic strength against pH inaqueous extract and pH₀ was recorded on a graphic ofΔpH against pH_H2O which gives de pH_o at the intersectionof pH_H2O axe. The adsorption study was performed bytriplicate, in isothermal conditions at 20 ± 2 ºC for 24 h,using batch equilibration procedure by treating 2 g ofcalcined substrate with 5, 10, 15, 20, 25, 30 and 40 mL of0.001 M Cu+2, in closed vessels. Then the Cu+2 equilibriumconcentration were determined by the complexometrictitration at pH 10 with a 0.001 M EDTA standard solutionand NET as metalochromic indicator. Thus, adsorptionisotherms were obtained by plotting the amount of copperadsorbed (mmol g-1 substrate) against the equilibriumconcentration (mmol) and fitted to the linear form of theLangmuir equation [15-17].

The pH and the EC variations were measured using the same batch equilibration procedure, in triplicate samples, by treating 2g of rawmaterial, activated and non-activated calcined substrate, with increasing volume of 0.001 M, 0.01 M and 0.1 M of Cu+2 solutions. Suspensions were periodically shaken at 20± 2 oC for 24 h in 100 mL glass beakers. pH was measuredwith a Hanna 211 pHmeter, calibrated with commercialbuffer solutions of pH 4 and 7. Electrical Conductivitywas measured with a Trans Instrument HC3010Conductimeter, calibrated with standard reference.

Determination of surface zero charge point

Amphoteric oxides have surface charges that are pH dependent; they react in alkaline or acid medium to create negative or positive charges which are responsible for the cation or anion adsorption reactions. So, the pH value at which surface positive charge equals to the surface negative charge is called the point of zero charge, PZC or pH₀ [18]. Figure 1 shows the results of the potentiometric titration on the raw material as well as on the nonactivated activated substrates. The intersection of the curves with the .H_H2O axis shows the pH value when the surface diffuses electrical charge equals to zero, indicating the PZC. All the curves present a single intersection point; therefore the PZC is defined by a unique pH value. For the raw material PZC is around 6 however, the Δ.H values are positive indicating that the surface charges are basically negative. However, in the case of calcined substrates surface charges varies according to the solution pH, indicating that the thermal treatment favors the formation of amphoteric oxides.

The PZC values for the calcined substrates lies between 6 and 7.2 and the PZC in calcined substrates shifts from 6.4 to 7 because of the alkaline treatment which creates a greater negative charge density on the substrate surface due to the oxides deprotonation reaction. The range in which the PZC of the raw material and the calcined substrates lies is similar to the PZC values for pure a-Fe₂O₃, goethite and gibbsite [19]. Moreover, the differences with these experimental values are probably are due to a mixture of amorphous variable charge oxides in the material. In reality, minerals and clays are rarely found in soils as pure mineral because particles can be formed of stratified layers of different mineral phases; clays could also be deeply associated with oxides and the amorphous material, making particular identification of minerals very difficult. Although it has been reported that mixing clays and minerals are very common in many types of soils [12].

Adsorption isotherm

Adsorption of copper ions takes place on active sites where amphoteric oxides are deprotonated, creating negative charges not only on non-activated but also in activated surface. Figure 2 shows the adsorption isotherm of Cu⁺² on adsorbent substrate activated with 0.10 N NaOH and non-activated. As it was pointed out in a previous paper [5], the L type isotherm indicates great affinity between copper ions and the substrate´s surface with the formation of a saturated monolayer of copper ions on the surface bounded probably through an inner sphere complex, as is predicted by the Langmuir model. The activated substrate enhances the adsorption reaction due to the grater negative charge density created after oxides deprotonation through alkaline treatment. The Isotherm profiles obtained from copper ions adsorption are similar to those reported from the Cu⁺² adsorption on goethite and g-Al₂O₃, and TiO₂ surfaces, confirming specific adsorption or chemisorption between Cu⁺² ions and oxide surface [20, 21]. The mechanism reported suggests the acidification of solution by the formation of H₃O. ions, which is showed by the pH measurement in different experimental conditions.

The adjustments to the Langmuir model was presented in the literature cited above [5] showing a good linear correlation and little average difference between experimental and calculated values. Table 1 shows the fitted equations and K values obtained for the adsorption reaction on the activated and non-activated substrate prepared with the granulometric fraction between 425 – 250 mm. The constant K. which defines the straight line slope is greater for the case of adsorption reaction on the activated substrate which agrees with the information given by the isotherm in the Figure 1.

However, isotherm may show but does not confirm information about the interaction between Cu⁺² ions and calcined substrate surface; neither does the FTIR spectra [5]. Nevertheless, L type isotherm is indicative of great affinity between copper ions and calcined substrate surfaces. Likewise, according to the literature, in a chemisorption reaction transitional metals should bond to the amphoteric surface through to the formation of a covalent bonding in an innersphere complex in which the metal becomes part of the oxide surface structure, according to the reaction 2 in which H₃O. ions are ones of the reaction products which acidifies the solution [3]. Unfortunately, the isotherm doesn´t explain the production of H₃O. ion during the adsorption reaction and cannot explain by itself the real interaction between copper ions and the substrate surface.

pH study

The information given by the isothermgraph and the fitting equations to the Langmuirmodel is insufficient to confirm the hypothesis of achemisorption reaction between copper ions andsubstrate surface. The theoretical model for thespecific adsorption or chemisorption type reactionbetween transitional metals and amphoteric oxides,described by equation (2), suggests the production ofH₃O+ ions during the adsorption reaction. Therefore,the pH measurement should provide the evidence ofan acidifying process during the adsorption reaction.Figure 3 shows the pH variation, by triplicating themeasurement, during the adsorption process as afunction of mmol of Cu+2 ions, added from a solution0.001 M, to the raw material, and calcined activated andnon-activated substrate. In all cases, copper adsorptionreaction produces acidification of the solution, becauseof H₃O+ ion production. However, acidification in thesolution is more accentuated in the non-activated andactivated substrates. The smaller acidification in theraw material has to be related to the smaller density ofnegative charges in the surface material.

Dotted, dashed and continued lines represent the three replicates measurements.

According to the information given by the isotherm graph and the pH study, it can be suggested that the interaction between copper ions and the amphoteric surface of the substrate can be explained by a chemisorption reaction according to the reaction (3): In any case, the interaction between copper ions and the oxidic surface must be strong enough because attempts for desorption even with strong acid solutions were infructuous.

Electrical conductivity study

Figure 5 shows the Electric Conductivity (EC) variation as a function of mmol of Cu+2 ions added to the raw material, and calcined activated and nonactivated substrate, using a 0.001 M Cu+2 solution. In all cases EC of the solution decrease due to the adsorption reaction. In the case of the raw material, there is a contribution to the EC from the dissolved fraction in the solution.

However, in the case of calcined substrates, the EC relay only on copper and sulphate ions, due to their greater concentration, other ionic species coming from the calcined substrate has minor contribution because calcined substrate is slightly soluble, so the dissolved fraction is negligible. On the other hand, the adsorption reaction on activate and non-activated substrates seems to take place by the same mechanism, but it is produced in a greater extend on activated material due to the creation of new activated sites through the deprotonation reaction. This aspect was also evidenced by the isotherms and Langmuir constants. The lowest EC values during adsorption reaction on activated substrate in relation to non-activated substrate shows that adsorption reaction actually takes place more extensively on this substrate.

Figure 7 shows Electrical Conductivity (EC) variation, of the triplicated measurements, as a function of mmol of Cu⁺² solutions added to the calcined substrate, activated in alkaline medium. The three graphics correspond to the 0.001 M, 0.01 M, and the 0.1 M Cu⁺² solutions used for the experiment. The EC decreases because of the adsorption reaction which is better defined at lower concentration where the EC decreases to 900 mS during the reaction, instead of 700 mS where the Cu^{+ 2} concentration is ten times greater and 150 mS when the concentration is 100 times greater.

The objective of this study is to take advantage of the physicochemical characteristics of the lithologic material found in the town of San Juan de Lagunillas, Mérida, Venezuela and uses it for the preparation of a calcined substrate that can be applied as a granular media for heavy metal retention in water treatment. Although more studies are required, the results obtained from previous works have shown that the lithologic material is appropriate for the preparation of ionic adsorbent substrates and their application in copper retention from aqueous solutions. Some of these results have shown great affinity between metal ion and adsorbent calcined surface characterized by an L type isotherm, and associated to chemisorption reaction between adsorbate and adsorbent. The mechanism of copper adsorption on oxidic surface seems to be the same for activated and non-activated surfaces however, the differences is given by the greatest density of negative charges created by the alkaline treatment, which deprotonates amphoteric oxides, enhancing adsorption reaction. Nevertheless, this evidence had to be complemented with additional data by proving the associated production of H₃O. ion to such kind of reaction. The pH measurements during the adsorption reaction showed a significant acidification along the reaction, which validates the literature about transitional metals chemisorption on amphoteric surface with variable charges. This acidification process became more intense as the concentration of copper ions in the solution increases; howere, the adsorption reaction is better defined atlow concentration.

The hypothesis of the chemisorptionreaction is also supported by the resistance of copper ionsto desorption reaction. This fact could be interpreted interms of a covalence formation between copper ions andthe calcined substrate in specific adsorption reaction. Itis expected that other transitional metals can suffer suchkind of reaction on the surface of these kinds of substrates,and it is possible to separate it from the contaminatedwaters during a filtration process in a granular media.The variable charge properties of the oxidic surface isevidenced not only by the PZC experiment, but also by theactivation reaction, which creates new negative chargesthat can participate in the adsorption phenomena, thusimproving the efficiency of calcined adsorbent substratefor ionic retention.

The amphoteric metallic oxides deprotonate in alkaline medium, increasing negative charge density on calcined adsorbent surface. The thermal treatment favors the formation of the amphoteric oxides with PZC similar to the reported values for pure Fe and Al oxides. The deprotonation reaction of the reactive groups in the substrate surface have been also proved by the EC measurements, showing that adsorption phenomenon is enhanced on activated surfaces with alkaline treatment just because Copper ions adsorb irreversibly on the amphoteric surface being unable to participate as mobile ions in the solution.