Adsorption of Cu2+ ions on humic acids

Geologia

Adsorption of Cu²⁺ ions on humic acids

Cláudio Pereira Jordão

Professor Titular do Departamento de Química - UFV

E-mail: [e-mail]

César Reis

Professor Adjunto do Departamento de Química - UFV

Carlos Roberto Bellato

Professor Adjunto do Departamento de Química - UFV

Gulab Newandram Jham

Professor Titular do Departamento de Química - UFV

José Luis Pereira

Bacharel e Licenciado em Química - UFV

Resumo

Amostras de ácidos húmicos de origem comercial (Fluka AG) foram usadas na avaliação do comportamento de adsorção de íons Cu²⁺. O modelo matemático descrito pela equação de adsorção de Langmuir em sua forma linear foi aplicado, obtendo-se os valores da capacidade máxima de adsorção b e da constante relacionada à energia de ligação a. Alíquotas de soluções de nitrato de cobre contendo várias concentrações desse metal foram adicionadas às amostras de suspensões de ácidos húmicos a pH constante. As isotermas apresentaram duas regiões distintas de adsorção, tanto a pH 4 como a pH 5, mostrando que o fenômeno adsortivo ocorre em duas etapas distintas, de acordo com o tipo de interação entre os ácidos húmicos e o cobre. As amostras de ácidos húmicos mostraram maior capacidade máxima de adsorção ao pH 5 do que ao pH 4, enquanto que a constante relacionada à energia de ligação foi maior em pH 4.

Palavras-chave: ácidos húmicos, cobre, adsorção, isoterma.

Abstract

Commercial samples of humic acids (Fluka AG) were used to evaluate the adsorption behaviour of Cu²⁺ ions. The mathematical model described by Langmuir's adsorption equation was applied and the values of the maximum adsorption capacity b and those of the constant related to the bonding energy a were obtained. Aliquots of copper nitrate solutions with different copper concentrations were added to the humic acid suspensions at constant pH values (4.0 and 5.0). The Langmuir isotherms presented two distinct adsorption regions, showing that the adsorptive phenomenon occurs in two distinct steps according to the copper-humic site interaction. Samples of humic acids showed higher maximum adsorption capacity at pH 5 than at pH 4, while the constant a related to bonding energy was higher at pH 4.

Key words: humic acids, copper, adsorption, isotherm.

1. Introduction

Humic substances play an important role in nature. The typical dark coloration of these substances increases heat retention by soils helping in seed germination. They also elevate the buffering and cation exchange capacities of soils. Due to the high water retention capacity (up to 20 times its mass), humic substances avoid drainage, being therefore important in soil conservation against erosion. They link to clays and minerals thereby cementing soil particles in aggregates, which increase the permeability and allow exchange of gases. The mineralization of organic matter results in the formation of NH₄⁺, NO₃^-, PO₄^3-, and SO₄^2- ions, which are important nutrients for plant growth. Another important property of humic substances is the capacity to interact with several metal ions to form complexes of different stability and structural characteristics. This capacity is due to the high content of functional groups containing oxygen, including carboxylic, phenolic hydroxyl and carbonyl (Stevenson, 1982; Tan, 1992).

Humic substances also interact with oxides, hydroxides, minerals and organic compounds to form compounds, which are soluble or insoluble in water, with different chemical and biological stabilities (Schnitzer, 1986). The formation of metal complexes and chelates with low solubilities, from the reaction between soil organic matter and metals, reduces the probability of a toxic metal contaminate the groundwater (Tan, 1992).

Discharges of wastes from some industries, as well as from the use of fungicidal sprays, may reach the soil surface causing environmental pollution with copper, which is found in soils mainly in divalent form. The major proportion of copper is associated with clays, iron hydroxides and organic matter, which control its concentration in the soil solution. The total soluble copper in acid soils is found as organic matter complexes (Malavolta, 1980; Sposito, 1989; Baker, 1993). Thus, the copper availability for plants depends on its characteristics of adsorption (Raghupathi & Vasuki, 1993).

The interactions of metals with humic acids extracted from marine sediments (Rashid, 1971), peat (Jordão, 1990) and soils (Kerndorff & Schnitzer, 1980) have been reported. Metal adsorption capacity changes with pH, concentration and type of metal. In order to express this behavior adsorption isotherms are commonly used. The most widely employed are those proposed by Langmuir and Freundlich (Cunha et al., 1994). The adsorption isotherms are widely used in the studies of interactions of heavy metals-soil-sediment. The Langmuir isotherms are particularly useful, since they furnish the values of the maximum adsorption capacity of the metal on the soil, and those of the constant related to the bonding energy of the adsorption process (Shuman, 1988; Duddrigde & Wainlwright, 1981).

The humic acids are one of the major soil constituents that play an important role in plants. Commercial humates have been comparable with soil humic acids in their effect on increasing the plant growth (Lobartini et al., 1992). Limited information is available on the origin and extraction methods of commercial humic acids. However, studies have shown that the interactions between metals and commercial humic acids (Fluka AG and Aldrich) are similar to those reported between metals and humic acids in soils (Beveridge & Pickering, 1980).

Thus, it is of great interest to study the adsorptive capacity of humic acids for essential nutrients and heavy metals, including copper. The objective of this work was to investigate the adsorption behaviour of Cu²⁺ ions by commercial humic acids using the mathematical model described by Langmuir equation.

2. Material and Methods

2.1 Adsorption of Cu²⁺ ions on humic acids

2.1.1 Adsorption as a function of time

Portions of 5 mg each of the humic acids sample were added to 50 mL polyethylene centrifuge tubes. A volume of 10 mL of copper nitrate solution containing 100 mg L^-1 of the ion was added to each tube. Electrodes were inserted and the pH values of the suspensions were adjusted to 5.0 ± 0.2 with 0.1 mol L^-1 NaOH solution. The volumes were adjusted to 30 mL with deionized water to obtain copper concentration of 33.33 mg L^-1. The tubes were mechanically shaken between 0-48 h at room temperature (24ºC-26ºC). After 1, 2, 3, 4, 5, 7, 10, 15, 19, 24, 36, and 48 h, the suspensions were centrifuged at 3,000 rpm for 20 minutes and filtered through a filter paper. The copper concentrations were determined in the solutions. The amounts of the adsorbed copper were calculated by the difference between the final and initial concentrations in the filtrates (Lamim et al., 1996).

2.1.2 Adsorption as a function of pH

Portions of 5 mg each of the humic acids sample were added to 50 mL polyethylene centrifuge tubes. A volume of 10 mL of copper nitrate solution containing 100 mg L^-1 of the ion was added to each tube. The pH of the suspensions was adjusted to 3; 4; 5; 6; 7; 8; 9; 10 and 11 (deviation of 0.2) with 0.1 or 0.01 mol L^-1 NaOH or HCl solutions. Volumes were adjusted to 30 mL with deionized water to obtain copper concentration of 33.33 mg L^-1. The centrifuge tubes were mechanically shaken for 2 h at room temperature (24ºC-26ºC). The pH of the suspensions was then measured. Further, the suspensions were centrifuged at 3,000 rpm for 20 minutes and filtered through a filter paper. The copper concentrations were determined in the solutions. The amounts of the adsorbed copper were calculated by the difference between the final and initial concentrations in the filtrates (Siqueira et al., 1989).

2.1.3 Evaluation of copperprecipitation

Aiming to observe precipitation of Cu²⁺ ions competing with adsorption process by humic acids, 10 mL aliquots of copper nitrate solution, containing 100 mg L^-1 of this element, were transferred to 50 mL polyethylene centrifuge tubes. The pH values of the solutions were adjusted between 5 and 7 with 0.1 mol L^-1 NaOH solution. The volumes were adjusted to 30 mL with deionized water to obtain copper concentration of 33.33 mg L^-1. Further, the resultant solutions were centrifuged at 3,000 rpm for 20 minutes, filtered through a filter paper and the concentration of copper determined (Costa, 1991).

2.1.4 Destructive Oxidation

In order to confirm the amount of copper retained on the humic acids after the adsorption process, the residue obtained was digested by oxidation for determination of Cu²⁺ ions in solution. For this purpose, portions of 5 mg each of the humic acids were added to 50 mL polyethylene centrifuge tubes. A volume of 10 mL of copper nitrate solution containing 100 mg L^-1 of the ion was added to each tube. The pH of the suspensions was adjusted to 5.0 ± 0.2 with 0.1 mol L^-1 NaOH solution. The volumes were completed to 30 mL with deionized water to obtain copper concentration of 33.33 mg L^-1. The suspensions were centrifuged at 3,000 rpm for 20 minutes at room temperature (24ºC-26ºC). The mixtures were filtered through paper filter when necessary. The copper concentrations of the solutions were then determined. The residues in the tubes were dried at 50ºC in a water bath, weighed and transferred to 250 mL beakers. Aliquots of 10 mL of 1 mol L^-1 HNO₃ and 5 mL of H₂O₂ solution (30 %, w/v) each were added and the mixtures further digested at 70ºC in a water bath. Finally, aliquots of 5 mL of H₂O₂ were also added to the mixtures, which were further cooled at room temperature (24ºC-26ºC) and filtered through a filter paper. The copper concentrations were then determined in the solutions (Siqueira et al., 1989).

2.1.5 The Langmuir absorption isotherm experiment

The linear form of Langmuir equation is given by C/x/m = 1/b.a+C/b, where for this experiment, C = the equilibrium concentration of copper solution in mg L^-1, x/m = the quantity of copper adsorbed in mg g^-1 of adsorbent, b = the adsorption maximum and a = constant related to the bonding energy of copper to the adsorbent. The coefficients a and b were obtained from the above equation. It furnishes a straight line, whose slope is the inverse of adsorption maximum and the intersection furnishing 1/b.a, from which the value of a is calculated (Msaky & Calvet, 1980). In order to conduct the experiment, portions of 5 mg each of the humic acids sample were added to 50 mL polyethylene centrifuge tubes. A volume of 10 mL of copper nitrate solutions varying between 0 and 450 mg L^-1 of the ion was added to each tube. The pH of the suspensions was adjusted to 4.0 ± 0.2 and 5.0 ± 0.2 with 0.01 mol L^-1 HCl or NaOH solutions. The volumes were adjusted to 30 mL with deionized water to obtain copper concentrations varying between 0 and 150 mg L^-1. The suspensions were mechanically shaken at 3,000 rpm for 20 minutes at room temperature (24ºC-26ºC). When necessary, the mixtures were filtered through a filter paper. The copper concentrations were then determined in the solutions. The amounts of the adsorbed copper were calculated by the difference between the final and initial concentrations in the solutions (Shuman, 1988).

2.1.6 Materials and Reagents

All glassware and materials were cleaned before metal analysis. All reagents were of analytical grade (Merck or equivalent). Filtration was carried out using filter paper Whatman, No. 541.

2.1.7 Instrumentation and determination

The pH values were determined with a model IRIS 7 Tecnow pHmeter equipped with electrodes of glass (indicator) and Ag-AgCl (reference).

The samples were centrifuged in a model 215 FANEM centrifuge.

Copper concentrations were measured with a Carl Zeiss JENA (model AAS3) atomic absorption spectrophoto-meter equipped by direct aspiration of the solutions into an air-acetylene flame.

Blanks were run through all experiments. All experiments were conducted in triplicates.

3. Results and discussion

Various studies have been conducted to verify the interaction of metal ions with humic acids suspensions (Jordão, 1990; Jordão et al., 1990; Jordão et al., 1993). In these studies, adsorption experiments were carried out using suspensions of humic acids (5 g L^-1). However, pipetting aliquots containing equal amounts of humic acid from their suspensions is difficult. Hence, in this study solid humic acids were utilised (5 mg portions). The formation of precipitates may interfere in the reaction of metals with humic acids. Hence, blanks were run to verify the precipitation of Cu²⁺ ions in absence of humic acids. A blue precipitate of copper hydroxide at pH 7 was observed. Calculations based on the Kps value of this compound showed that copper concentrations used in this study would lead to precipitation of Cu²⁺ ions at pH close to 5.5. Although the precipitate could not be visualised at this pH, this value was not used in the adsorption experiments in this study (the chosen pH values selected were 4 and 5). On the other hand, the digestion by oxidation of the residue conducted after the adsorption to confirm the copper concentration retained in the humic acids showed that on an average, 95 % of the Cu²⁺ ions were in the adsorbent.

The results of the influence of shaking time on the reaction between Cu²⁺ ions and humic acids are shown in Figure 1. A large variation was observed in the percentage of adsorption according to the shaking time of the suspensions. In order to study the influence of the pH on the adsorption of Cu²⁺ ions by humic acids, a 2 h period of shaking was selected. It seems reasonable to choose this value since the adsorption occurred at a relatively elevated degree in relation to the total period (48 h). In related studies, periods of shaking varying from 2 h to 16 h have been applied to the adsorption of various metal ions, including Cu²⁺, by humic acids extracted from soils (Kerndorff & Schnitzer, 1980) as well as commercial humic acids (Beveridge & Pickering, 1980).

Figure 1
- The effect of time on Cu adsorption by humic acids.

The choice of pH to study the metal adsorption on to humic acids is based on the availability of the element in solution, as well as in the amount of surface charges of the solid phase. At very low pH values, there is a competition for the active sites of macromolecules between metal ions and H⁺ ions from the solution. As the pH rises, the degree of competition decreases and the metals become more easily retained until the adsorption maximum is reached. Thus, the copper concentration in solution increases at low pH values, while there is a decrease of the negative charges of the adsorbent surface through retention of protons. This makes adsorption difficult at pH values below 6.

The adsorption curve of Cu²⁺ ions in humic acids is bell-shaped (Figure 2). A sharp reduction was observed in the adsorption from pH 9 to 11, certainly due to dissolution of organic matter and consequent formation of soluble copper complexes (Beveridge & Pickering, 1980). High adsorption values between pH 7-9 could be attributed to the adsorption of Cu²⁺ ions by humic acids. In spite of the fact that the adsorption progressively increased up to pH 7, the experiments to obtain the Langmuir isotherms were conducted at pH 4 and 5, since these values are closer to the pH of acid soils commonly found in Brazil.

Figure 2
- Effect of pH on Cu adsorption by humic acids.

Several studies have shown that the adsorption of metal ions by soils and sediments follow adsorption isotherms of Freundlich and Langmuir (Cunha et al, 1994; Duddrigde & Wainlright, 1981). However, only few studies have been applied to the adsorption of Cu²⁺ ions by commercially available humic acids using Langmuir isotherms (Tu et al., 1993). Thus, the lack of adequate information does not permit specific conclusions. Copper adsorption fitted to the Langmuir isotherm and two linear portions (hereafter referred to as first and second region) of the curve were found (Figure 3). Similar results have also been reported for zinc in soils (Shuman, 1975). This fact is based on the hypothesis that each linear portion is related to different types of adsorption sites of the adsorbent. Thus, the adsorptive capacity and the relative bonding energy can be calculated (Harter & Smith, 1981). Hence, the adsorption sites were separated in first and second regions (Figure 3).

Figure 3
- Langmuir isotherm for Cu adsorption by humic acids at pH 5: (A) 1^st region of adsorption; (B) 2^nd region of adsorption.

Table 1 shows that the adsorption maximum b of humic acids for copper at pH 5 was greater in the second region than the first one. However, the value of the coefficient related to the bonding energy a was higher in the first region than the second. The Cu²⁺ ions are strongly bonded to the humic acids in the first region due to the greater availability of reaction sites. High-energy value suggests that in the first region, the Cu²⁺ ions are retained, for instance, by complexation. In contrast, the coefficient related to the bonding energy is lower in the second region as compared with the first one, which may suggest electrostatic or even van der Waals interactions.

Table 1
- Coefficients of Langmuir equation for adsorption of Cu²⁺ ions on humic acids according to the adsorption region and pH.

The results show that the Langmuir adsorption isotherm obtained at pH 4 was similar to that at pH 5. Thus, two linear portions were obtained (Figure 4) as well as higher adsorptive capacity b and smaller bonding energy coefficient a in the second region of the curve than in the first one (Table 1). The adsorption maximum capacity of the humic acids for copper was higher at pH 5 as compared with the value at pH 4. However, a relatively high bonding energy coefficientwas found at pH 4 in the first region of the curve, which could mean that Cu²⁺ ions were not easily liberated to the solution at this pH value.

Figure 4
- Langmuir isotherm for Cu adsorption by humic acids at pH 4: (A) 1^st region of adsorption; (B) 2^nd region of adsorption.

The adsorption maximum values of several metal ions, including Cu²⁺, by humic acids from Fluka AG and Aldrich Chemical Co., at pH 5, were 51.47 and 33.68 mg g^-1, respectively (Beveridge & Pickering, 1980).

4. Conclusions

The data on Cu²⁺ ion adsorption by humic acids of commercial origin conformed to the linear form of the Langmuir isotherm. At pH values of 4 and 5, the Langmuir isotherm presented two linear regions of the curve. At both pH values, the second region presented higher adsorptive capacities b and lower values of the related bonding energy coefficient a than the first region. These results suggest that the high values found for the coefficient related to the bonding energy in the first region can be related to the interactions of relatively high energies such as complexation. The lower values of the relative bonding energy of copper for the humic acids in the second region suggest weaker bonds such as electrostatic or even van der Waals interactions.

Considering similarities between commercially available humic acids and those extracted from the soils, one can conclude that at pH values commonly found in acid soils, the humic acids adsorb high concentrations of Cu²⁺ ions, making it difficult to be liberated to the soil environment. The adsorption process is thus very important to control additions of ions, when micronutrients or heavy metals are present in high concentrations in soils.

Artigo recebido em 08/02/2001.

• BAKER, D. E. Copper. In: ALLOWAY, B. J. ed. Heavy metal in soils Glasgow: Blackie Academic & Professional, 1993. 339 pp., p. 151-176.
• BEVERIDGE, A., PICKERING, W. F. Influence of humate-solute interactions on aqueous heavy metal ion levels. Water, Air and Soil Poll., v. 14, p. 171-185, 1980.
• COSTA, E. D. Adsorção e competição de alguns metais por ácidos húmicos extraídos de um latossolo húmico da região de Araponga, Minas Gerais. Viçosa: Universidade Federal de Viçosa, 1991. 71 p. (Dissertação de Mestrado).
• CUNHA, R. C., CAMARGO, O. A., KINJO, T. Aplicação de três isotermas na adsorção de zinco em oxissolos, alfissolos e ultissolos. Rev. Bras. Ciênc.Solo, v. 18, p. 15-20, 1994.
• DUDDRIGDE, J. E., WAINLWRIGHT, M. Heavy metals in river sediments_calculation of metal adsorption maxima using Langmuir and Freundlich isotherms. Env.Poll., v. 2, p. 387-397, 1981.
• HARTER, R. D., SMITH, G. Langmuir equation and alternate methods of studying "adsorption" reactions in soils. In: Chemistry in the soil environment. Madison: American Society of Agronomy, 1981, n. 40, p. 167-182. (Asa Special Publication).
• JORDÃO, C.P. Interação de cádmio, cobre, chumbo e zinco com ácidos húmicos extraídos de turfa. Rev. Ceres, v. 37, p. 72-84, 1990.
• JORDÃO, C. P., SIQUEIRA, R. N. B., GOULART, A. T., BRUNE. W. Adsorção competitiva de cobre, chumbo, zinco e cádmio por ácidos húmicos em suspensões aquosas. Ciênc. e Cultura, v. 42, n. 10, p. 797-801, 1990.
• JORDÃO, C. P., COSTA, E. D., BRUNE, W., GOULART, A. T. Adsorção de cátions metálicos em ácidos húmicos de latossolo. Quím. Nova, v. 16, n. 6, p. 517-520, 1993.
• KERNDORFF, H. & SCHNITZER, M. Sorption of metals on humic acid. Geoch.et Cosm.Acta, v. 44, p. 1701-1708, 1980.
• LAMIM, S. S. M., JORDÃO, C. P., BRUNE, W., PEREIRA, J. L. Caracterização de vermicomposto de esterco bovino e avaliação da adsorção competitiva por cádmio e chumbo. Quím. Nova, v. 19, n. 5, p. 457-463, 1996.
• LOBARTINI, J. C., TAN, K. H., REMA, J. A., GINGLE, A. R., PAPE, C., HIMMELSBACH, D. S. The geochemical nature and agricultural importance of commercial humic acid. Sci. Total Env., v. 113, p. 1-15, 1992.
• MALAVOLTA, E. Elementos de nutrição mineral de plantas. São Paulo: Agronômica Ceres, 1980. 252 p.
• MSAKY, J. J., CALVET, R. Adsorption behaviour of copper and zinc in soils: influence of pH on adsoption characteristics. Soil Sci., v. 150, p. 513-522, 1990.
• RASHID, M. A. Role of humic acids of marine origin and their different weight fractions in complexing di- and tri-valent metals. SoilSci., v. 111, p. 298-306, 1971.
• RAGHUPATHI, H. B., VASUKI, N. Copper adsorption of some soils of north Karnataka. J. Ind. Soc. Soil Sci., v. 41, p. 70-74, 1993.
• SCHNITZER, M. Interactions of soil minerals by organic acids. HUANG, P. M., SCHNITZER, M. ed. SSSA Special Publication, 1986. n. 17, p. 77-101.
• SHUMAN, L. M. Effect of soil properties on zinc adsorption by soils. Soil Sci.Soc.Amer.J., v. 39, n. 3, p. 454-458, 1975.
• SHUMAN, L. M. Effect of removal of organic matter and iron- or manganese-oxides on zinc adsorption by soil. Soil Sci., v. 146, n.4, p. 248-254, 1988.
• SIQUEIRA, R. N. B., JORDÃO, C. P., GOULART. A. T., BRUNE, W. Extração seletiva de cádmio, chumbo, cobre e zinco pre-adsorvidos por ácidos húmicos. Quím. Nova, v. 12, n.1, p. 9-13, 1989.
• SPOSITO, G. The chemistry of soils. New York: Oxford University Press, 1989. 69 pp.
• STEVENSON, F. J. Humus chemistry. New York: John Wiley & Sons, Inc., 1982. 443 pp.
• TAN, K. H. Principles of soil chemistry. New York: Marcel Dekker, Inc., 1992. 362 p.
• TU, Q., SHAN, X. Q., NI, Z. M. Comparative characteristic studies on soil and commercial humic acids. FreseniusJ.Anal.Chem., v. 347, p. 330-336, 1993.

Publication Dates

• Publication in this collection
  22 Aug 2003
• Date of issue
  June 2001

History

• Received
  08 Feb 2001