Adsorption of sulphuric acid on smectite from acidic aqueous solutions

Adsorção de ácido sulfúrico em esmectita de soluções aquosas ácidas

Adsorption of sulphuric acid on smectite from acidic aqueous solutions

(Adsorção de ácido sulfúrico em esmectita de soluções aquosas ácidas )

E. L. Tavani , C. Volzone

Centro de Tecnología de Recursos Minerales y Cerámica (CETMIC)

Comisión de Investigaciones Científicas de la Provincia de Buenos Aires

Consejo Nacional de Investigaciones Científicas y Técnicas

C.C. 49, (1897) M.B. Gonnet, Argentina

Abstract

The adsorption of sulphuric acid on smectite from acidic aqueous solutions was studied. The amounts of cations dissolved in each equilibrium solution were determined by chemical analysis. Simultaneously, the original smectite and the smectite after each test were characterized by infrared, X-ray diffraction and swelling index. The results obtained permitted us to determine that the substitutions of the exchange cations and the chemical attack occurred at very different acid concentrations and treatment times. It was established that during the acid treatment of the smectite, a progressive damage of its crystalline structure was produced. Finally, the influences of the substitution of exchange cations and of the chemical attack in the swelling index were determined.

Keywords: adsorption, sulfuric acid, smectite, clay liner.

Resumo

A adsorção de ácido sulfúrico em esmectita de soluções aquosas ácidas é estudada. As concentrações de cátions dissolvidos em cada solução em equilíbrio foram determinadas por análise química. As esmectitas original e após cada teste foram simultaneamente caracterizadas por infravermelho, difração de raios X e índice de crescimento. Os resultados obtidos permitiram determinar que as substituições dos cátions de troca e o ataque químico ocorreram em diferentes concentrações de ácido e tempos de tratamento. Foi verificado que durante o tratamento em ácido da esmectita foi produzido um dano progressivo da sua estrutura cristalina. Finalmente foram determinadas as influências da substituição dos cátions de troca e do ataque químico no índice de crescimento.

Palavras-chave: adsorção, ácido sulfúrico, esmectita, revestimento de argila.

INTRODUCTION

The sulfuric acid is used in the mining industry as a chemical reagent to remove impurities contained in ores and to dissolve those useful minerals of the concentrates. Several studies were performed to decrease the content of acid in liquids discharged during operations and processes of mineral concentrations and in the obtention of metals [1-4]. In all of these cases, the purpose was to find the minimum amount of acid in the effluent.

The presence of sulfuric acid in an effluent is not always related to its use as chemical reagent. Thus, the acid may be formed from a chemical and bacterial oxidation of sulfur-bearing minerals, such as pyrite, pyrrhotite and chalcopyrite [5-6]. The effluent resulting from this oxidation has low pH and high metal content. The development of inhibitors to prevent the chemical and bacterial oxidation is an alternative to control the acid formation [5].

Clays are substances with high adsorption capacity of metals and low permeability (hydraulic conductivity). Clays may be grouped according to their chemical composition and crystalline structure in kaolinites, illites and smectites. Of these three types, smectites have higher adsorption capacity and lower permeability [7-10].

Clays are frequently found in nature above and/or below an aquifer [7]. The low permeability of the clay impedes or reduces the passage of liquid effluents through that material and its high capacity of adsorption immobilizes the metals contained in such effluents. Taking into account these facts, the clays are added to the bottom and sides (clay liner) of a pit designed for use as a disposal site for potentially dangerous waste [7] and to perform the total covering of tailing (mine deactivation) to avoid water passage and oxygen access [6].

The sulfuric acid is a chemical reagent that may produce the dissolution of a clay. The chemical attack starts with the acid adsorption on the solid surface which leads to the substitution of the exchange cations by protons. Then, the adsorbed protons diffuse towards the active sites of the reactive solid, the chemical reaction is produced (breakage of chemical bonds and formation of new bonds) and soluble reaction products are desorbed into the liquid phase [8-10].

In order to determine the danger of an effluent containing sulfuric acid, it was considered as appropriate to know the changes of the physicochemical features of a clay after substitution of the exchange cations by protons and after desorption of the soluble reaction products. According to the above mentioned, the aim of this work was to study the adsorption of sulfuric acid on smectite (clay with low permeability and high capacity of adsorption) from acidic aqueous solutions.

EXPERIMENTAL

Adsorption tests were carried out with sulfuric acid solutions of 0.02 and 2.00 N concentrations during different times. The smectite was previously dispersed in distilled water (5 % w/w) and in this condition it was added to the acidic aqueous solution. In all cases, the smectite/acidic aqueous solution ratio was 0,25 % p/v. The addition of the clay suspension was performed slowly while the new system (acidic aqueous solution + clay suspension) was maintained with stirring. When the addition of the suspension ended, the stirring was interrupted and the mentioned system was maintained at a constant temperature (T= 20 °C). After 1; 6; 12 and 24 hours the solids were separated by centrifugation and then washed with distilled water to remove excess electrolyte and soluble salts. Finally, the solids so obtained were dried at room temperature and weighed for their subsequent physicochemical characterization.

The amounts of sodium, potassium, calcium and magnesium (exchange cations) and of aluminum and magnesium (chemical attack) corresponding to each treatment with sulphuric acid were determined by chemical analyses of the equilibrium solutions. Chemical analyses were carried out by atomic absorption/emission (AA/AE), titrimetry and gravimetry. The AA/AE analyses were made with a Jarell Ash equipment.

The smectite sample was supplied by Georgia Kaolin Co., USA. The presence of smectite was confirmed by the d(001) spacing of the sample after air drying, calcination at 600 °C during 2 hours and glycol treatment. From d(060) spacing it was determined that the smectite used in this work was essentially dioctahedral. By means of specific tests, montmorillonite (95 %) and beidellite (5 %) were identified. The content of impurities was lower than 7 % (quartz and cristobalite). More than 90 % of the sample had a particle size smaller than 2 mm. The cationic exchange capacity (CEC) of the sample was 0.853 meq/g of smectite and it was measured by the ammonium acetate technique using phenolphthalein as indicator.

The physicochemical characterization of the smectite before and after each acid treatment was performed by infrared (IR), X-ray diffraction (XRD) and swelling index (SI).

The IR spectra were obtained with a Perkin-Elmer spectrophotometer, model 577, with a scanning range between 4000 and 400 cm^-1. The samples were prepared as tablets diluted in KBr, keeping constant the mineral/KBr ratio and the total weight of sample.

The XRD analyses were made with a Philips equipment, model 1140/00, using Cu-Ka radiation (0.1542 nm) and a Ni filter. The spacing and the width corresponding to the reflection from the (001) planes of each sample were measured by scanning at 1° (2q)/min, between 3 and 12° (2q), on oriented specimens. The width was determined, in degrees (2q), at an intensity equal to half the maximum intensity [11].

The SI values were determined by addition of 2 g of dry smectite to 100 ml of distilled water. After 24 hours the volume of sediment was read. The results obtained were relative values in order to compare the behavior of different samples.

RESULTS AND DISCUSSION

Table I shows the amounts of exchange cations dissolved in the equilibrium solution of the test carried out with 0.02 N sulfuric acid during 1 hour. The total amount of dissolved cations was 15 % higher than the CEC determined for the sample without acid treatment (0.853 meq/g of sample). The difference between values would be caused by the different pH at which both determinations were performed and by the soluble salts contained in the original sample. Tests were repeated during 6; 12 and 24 hours and in all cases the values obtained for each cation were similar. Finally, the aluminum presence was not determined in any of the equilibrium solutions of the tests performed with 0.02 N sulfuric acid (1-24 hours).

The amounts and nature of cations dissolved in equilibrium solutions of tests carried out with 2.00 N sulfuric acid showed some changes with the increase of the treatment time. In the test performed with 2.00 N acid during 1 hour, the amounts of dissolved exchange cations were similar to the ones indicated in Table I (0.02 N acid during 1 hour). However, this behavior changed for tests performed during 6; 12 and 24 hours. The aluminum presence and a higher magnesium content in the equilibrium solutions of these tests were evidences used to prove the occurrence of a chemical attack to the smectite. Thus, in the test carried out during 24 hours the following amounts were determined: 0.55 meq of aluminum/g of sample and 0.10 meq of magnesium/g of sample while the other cations (sodium, potassium and calcium) remained constant. The obtained results were analyzed from the type of structure of the smectite and from the characteristics of the acid reagent.

The natural smectite has a 2:1 layer structure (one aluminum octahedral sheet between two silicon tetrahedral sheets) and a net negative charge as a result of isomorphic substitutions in the tetrahedral sheets (Si⁴⁺ ions in the basic structure are replaced by Al³⁺ions) and in the octahedral sheets (Al³⁺ions are replaced by Mg²⁺ ions). This net negative charge is balanced by exchange cations (Na⁺, K⁺, Ca²⁺, Mg²⁺) that are primarily adsorbed in the interlayer space. Under certain conditions, the cations that balance the net negative charge may be exchanged by other cations.

The cations in aqueous solution are not as separate entities. The high field strength of the cation immobilizes the nearest-neighbor water molecules as a result of the cation-water attraction (hydration). When pure sulfuric acid is diluted with water, its dissociation occurs (H₂SO₄ + H₂O « H₃O⁺ + HSO₄^- ). A high water content is required for the occurrence of the second stage of dissociation (HSO₄^- + H₂O « H₃O⁺ + SO₄⁼ ).

After treatment with 0.02 N sulfuric acid, the exchange cations of the original sample were substituted by H₃O⁺. The small size (r) of H₃O⁺ and its high ionic potential (electric charge of hydrated cation/r) facilitates its penetration into the crystal of smectite. However, a small fraction of the adsorbed H₃O⁺ may enter in its interior to displace aluminum and magnesium [8-10]. Consequently, a high acidity of the solution favors the occurrence of the mentioned stages which would explain the results obtained with the two acid concentrations used in this work.

The IR spectrum of the original smectite showed the following bands: 3620; 3425; 1034; 914; 835; 794; 523 and 464 cm^-1. All bands remained after treatments with 0.02 and 2.00 N sulfuric acid even though almost all of them changed their intensities. This behavior would indicate a minimum damage of the crystalline structure of smectite which showed a reasonable agreement with the dissolved amounts of aluminum and magnesium determined in the equilibrium solutions.

The stretching band at 3620 cm^-1 is due to OH groups and the stretching band at 3425 cm^-1 has been assigned to OH groups bonded to adsorbed water. Both bands showed minimum intensity changes after acid treatment. The stretching band at 1034 cm^-1 (Si-O-Si) shifted to a higher frequency (1079 cm^-1) with a simultaneous widenning. The stretching bands at 914 cm^-1 (Al₂-OH) and 835 cm^-1 (Mg-Al-OH) showed a slight intensity decrease for the smectite treated with 0.02 and 2.00 N sulfuric acid. On the contrary, the stretching band at 794 cm^-1 (Si-O-Si) showed an increase of its intensity. Finally, the bending bands at 523 cm^-1 (Si-O-Al) and 464 cm^-1 (Si-O) decreased their intensities, resulting a higher decrease in the first band.

Table II shows the spacings and widths of the original smectite and of the smectite treated with 0.02 and 2.00 N sulfuric acid. The results obtained by us showed that the separation between layers of the smectite treated with 0.02 and 2.00 N sulfuric acid was the same as the separation of the original smectite, whereas the width of reflections increased with the acid concentration and the treatment time. Both aspects (spacing and width) were measured to obtain evidences with regard to changes of crystalline structure and atomic arrangement of the smectite produced by the treatment with sulfuric acid.

In the smectite, the ensemble of elementary layers is ordered along the c-axis. The separation between layers in this lamellar clay may be determined from d(001) spacing. Taking into account that each interlayer cation touches two smectite layers, it is possible to say that the exchange of such cations would produce a change of d(001) spacing.

The spacing of the original smectite was 1.53 nm and showed an acceptable agreement with the expected value according to the exchange cations contained in the original sample. The spacings corresponding to the monoionic forms of smectite taken as reference for this analysis were: 1.33 nm for Na-smectite; 1.22 nm for K-smectite; 1.42 nm for Mg-smectite and 1.69 nm for Ca-smectite [12-13].

The d(001) spacing of the smectite treated with 0.02 N sulfuric acid was similar to the value determined for the monoionic form of H-smectite in the works above mentioned [12-13]. This behavior would indicate, as it was stated when we referred to the results obtained by chemical analysis, that the substitution of the exchange cations by H₃O⁺ was produced. The maintenance of the same d(001) spacing for the treatments made with 0.02 and 2.00 N sulfuric acid would be produced because in both cases the interlayer space was occupied by H₃O⁺.

The width of the (001) reflection permits to study the way in which the laminar habit (crystallinity) of the smectite changes with the acid treatment. Thus, a smectite with a good crystallinity originates a narrow (001) reflection. On the contrary, when the crystallinity decreases a reflection widening is produced. Results obtained in this work showed that a reflection widening was produced which would indicate that the crystallinity of the smectite treated with 0.02 and 2.00 N sulfuric acid decreased.

Table III shows the SI of the original smectite and of the smectite treated with 0.02 and 2.00 N sulfuric acid. The swelling of a clay occurs between layers within particles and between particles. This behavior depends on the amount of surface charge and on the nature of the exchange cations. The swelling of the smectite is potentially reversible in the presence of some substances. The reversible nature of this swelling may result in hydraulic conductivity increases which affect the performance of a clay liner [7]. The SI was measured to know this behavior.

The volume of 19 mL determined for the original smectite showed a reasonable agreement with the content and nature of the exchange cations, whereas the SI for the smectite treated with 0.02 N sulfuric acid decreased up to 6 ml and did not change with the treatment time. The volume of 6 ml was similar to the value determined in a previous work for the monoionic form of H-smectite [14] and it would confirm, as it was previously mentioned, that the cation placed into the interlayer space was H₃O⁺.

The SI values for the smectite treated with 2.00 N sulfuric acid were slightly higher than the value of the smectite treated with 0.02. This behavior would be produced by the loss of laminar habit caused by the acid treatment. The loss of laminar habit would have an effect similar to the one observed during the mechanochemical treatments of the mentioned monoionic form of H-smectite.

CONCLUSIONS

The adsorption of sulfuric acid on smectite led to the substitution of the exchange cations by H₃O⁺ and to the subsequent chemical attack of the clay. The adsorption of H₃O⁺ was produced from an acidic aqueous solution of low concentration (0.02 N during 1 hour) while the chemical attack was produced under more severe conditions (2.00 N during 6 hours). Both events caused the dissolution (mobilization) of the exchange cations adsorbed on the original sample and of the soluble reaction products.

The IR spectra showed that the damage of the crystalline structure was low with the acid concentrations and treatment times used in this work. It was determined by XRD that a progressive loss of the laminar habit of the smectite was produced, whereas the c-dimension of the unit cell did not change.

The swelling index decreased after substitution of the exchange cations and increased with the chemical attack. These results show that the substitution of exchange cations and the subsequent chemical attack of a smectite would affect its behavior when it is used as clay liner.

(Rec. 25/08/98, Ac. 12/03/99)

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Publication Dates

• Publication in this collection
  31 Mar 2000
• Date of issue
  May 1999

History

• Accepted
  12 Mar 1999
• Received
  25 Aug 1998