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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.3 Concepción sep. 2003

http://dx.doi.org/10.4067/S0717-97072003000300018 

J. Chil. Chem. Soc., 48, N 3 (2003) ISSN 0717-9324

CHARGE DEVELOPMENT AND ACID-BASE CHARACTERISTICS
OF SOIL AND COMPOST HUMIC ACIDS

P. A. CAMPITELLI, M. I. VELASCO, and S.B. CEPPI*

Dept. De Recursos Naturales, Facultad de Ciencias Agropecuarias, Universidad Nacional de Córdoba,
Córdoba, Argentina
E-mail: sceppi@agro.uncor.edu

ABSTRACT

In previous works, the acid-base properties, charging behavior and chemical heterogeneity of humic substances have been studied using different mathematical equations to fit the experimental data.

The objective of this research is to study the charge behavior, acid-base properties and analyze the chemical heterogeneity of humic acids (HA) extracted from soil and composted municipal solid waste by potentiometric titrations.

The humic acids extracted from compost have some characteristics and behavior similar to those obtained from soil.

The negative charge development of HA extracted from composted material are lower than those extracted from soil and increase as ionic strength increase. The amount of carboxylic groups is lower in compost HA than in soil HA The heterogeneity of HA extracted from compost is higher than those extracted from soil.

As the time of composting period increase the humification processes that take place trends to produce compost HA that has similar characteristics to soil HA. We suggest that HA extracted from composted material are macromolecules "like soil humic acids", i.e. "humiclike fraction".

Key words: compost, humic acids, charge development, acid-base characteristics.

INTRODUCTION

Humic substances (HS) are complex and ill-defined polydispersed mixtures of heterogeneous polyelectrolytes, and are important in natural environment for their role in regulating ion concentrations in solution. This is especially true with metal ions that bind strongly to these organic materials (1). The metal binding capacity of humic substances finds its origin in the presence of acidic functional groups. Humic substances are chemically heterogeneous and differ from single ligands in the number of different complexing sites per molecule. The binding is influenced by the variable charge properties and the chemical heterogeneity of the humic material. As a consequence, ion binding to these materials depends not only on the ion concentration itself but also on environmental conditions, such as pH and ionic strength (2, 3).

In order to gain understanding about the binding mechanism and to develop a model for ion binding under different conditions, the charge behavior of humic materials should be known. The charging behavior can be studied by performing acid/base titration on the humic substances in absence of specifically adsorbing ions. In this way charge-pH curves for different salt levels can be obtained.

The interpretation of charge-pH curves is a delicate problem. Due to the large variety of different functional groups, humic substances should be considered as heterogeneous ligands. In this way, the polymer has a number of acidic groups, which are chemically non-identical but whose acid strengths are so similar as to prevent the appearance of separate titration end points. Other complicating factors are the polydispersity of humic substances, their polyelectrolyte behavior, conformational changes, and electrostatic effects (3, 4), which makes difficult the interpretation of titrations curves in terms of pKa values.

Humic substances constitute a substantial fraction of the organic matter in soils, sediments, water, and organic amendment materials such as composted urban solid waste or composted sewage sludge. It is necessary to note that the expression humic substances and humic acids have only an operational meaning when applied to newly formed organic materials (i.e. the organic matter soluble in alkali but precipitated by mineral acids) These substances contain appreciable concentration of acidic functional groups. Most of these acidic functional groups are carboxylic and phenolic acids (1).

The chemical properties of the acidic functional groups present in humic substances extracted from soil or composted materials are believed to be of central importance to an understanding of how the chemistry of the soil solution may change after the application of composted materials to agricultural lands (5, 6).

In previous works, the acid-base properties, charge behavior and chemical heterogeneity of humic substances have been studied using different mathematical equations to fit the experimental data in order to obtain knowledge in this field (3, 7-14).

The slope of the charge-pH (Q vs. pH) curve is related to the degree of heterogeneity (i.e. the smoother the Q vs. pH curve is, the lower is the heterogeneity of the sample), the identity of the acidic functional groups and the apparent affinity distribution (3, 7-10). The slope is determined by the first derivative of the charge-pH curves (dQ/dpH). For smoothing purposes, Machesky (9) proposed a polynomial equation to fit the experimental net titration data (corrected for blank solution effect) of the form:

cmol of H+ bound or released/ kg of HA(Q) = a(pH)n +
b(pH)n-1++f(pH) + g

The first derivatives are obtained on the bases of a smoothing function through the experimental data. The first derivatives generated with the actual experimental data were similar in shape and magnitude but "noisier" than the smoothed data.

The buffer capacity (b) of the acid-base system is inversely proportional to the slope of the titration curve at a particular pH and is defined as:

b = dCa/dpH º dQ /dpH

where Ca is a number of moles/l of strong acid required to produce a change in pH of dpH. This function was used to locate buffer capacity maximum and minima for humic substances by Machesky (9). The buffer capacity values may be calculated by solving for the first derivatives of the polynomial regressions (dQ/dpH), where Q is defined as cmol of H+ bound or release by kg of humic acid. Thus, the first derivative of the smoothing function can also be used to estimate the pH at which a particular acidic functional group is half-titrated. De Wit et al., (2) utilize buffer capacity distribution to derive the apparent distribution for the acidic functional groups of humic substances.

Nederloff et al., (3) proposed the use of the first derivative of the charge-pH curve (dQ/dpH) for the heterogeneity analysis. These derivatives are also obtained on the basis of a smoothing function (polynomial equation) through the experimental net titration data.

The distribution can be interpreted as a probability density function. That is, the curves indicate the probability of finding charged sites with an affinity in the range pH + dpH (Kapp + dKapp). When the affinity distribution shows well separated narrow peaks, the surface is characterized by a few discrete sites. The number of sites classes is equal to the number of peaks, and the apparent affinity constant values follow from the peak positions. When the distribution is wide and smooth, the sample is characterized as a continuous heterogeneous ligand ( 2, 3).

With this derivative function we obtain the distribution function F proposed by de Wit et al. (8), and Nederlof et al. (3,10):

F (log KHapp) = (dQ /dpH)

where log KHapp is the apparent affinity constant of the protonation reaction for a certain group. In this way, it is possible to obtain information, or trends can be predicted, about charge development, apparent affinity constants of the protonation reactions, negative sites heterogeneity and buffer capacity distribution.

The distribution function F is a non-normalized distribution, which can be normalized if Qmax is known. Qmax is, however, very difficult to determine experimentally. Normalization changes neither the location of the distribution on the log Kapp (or pH) axis, nor the shape of the distribution function (8).

The objective of this research is to study the charge behavior, acid-base properties and analyze the chemical heterogeneity of humic acids extracted from soil and composted municipal solid waste using potentiometric titration.

EXPERIMENTAL

Materials

The humic acid (HA) fraction from soil and compost were extracted with NaOH 0.1 M, purified with HCl:HF (1:3) and finally dried at low temperature, according to the procedure recommended by Chen et al. (15).

The HA samples used in this work were:

1) HA extracted from an Entic Haplustoll of Province of Cordoba-Argentina

Denoted as: HA-S (Entic Haplustoll).

2) HA extracted from composted solid urban waste of two cities from Province of Cordoba-Argentina: Oncativo and La Para.

Composting process in Oncativo was performed in piles, mixing solid urban waste with low content of vegetal materials, in open air with occasional turning every 10 or 15 days. The Oncativo samples were extracted after two periods of composting: 6 and 14 month.

In La Para, the composting procedure was the same, but it also included the addition of cattle manure (20-30% v/v) and a high soil content (@ 30-40% v/v) in the composting mixture. The composting time was approximately 8-9 month.

The HA obtained are denoted as: a) HA-O6 (HA extracted from Oncativo composted 6 month); b) HA-O14 (HA extracted from Oncativo composted 14 month) and c) HA-LP (HA extracted from La Para compost).

Methods

A stock of AHs solution was prepared by dissolving AHs with a minimun volume of Na(OH) solution and adding water up to final volume. An aliquot containing the desired amount of humic acids (@ 9 mg) was transferred to the titration flask that contains 10 ml of double distilled water. The titrant (HCl @ 0.05N) was added from an automatic burette (Schott Geräte T80/20) at a titration rate of 0.1 ml / 40 s. This rate was chosen taking into consideration that the variation of pH values should range between 0.02 and 0.04 pH units. The pH values were measured with an Orion research 901 pH meter equipped with a glass-combined electrode Orion 9113.

All titration were performed in absence (I=0 M) and presence (I=0.1 M) of NaCl background electrolyte. A reference or blank titration was carried out for each titration curve in order to correct the HA charge development for dilution effects.

Total acidity and carboxylic group (COOH) contents were determined according to conventional methods (16), and phenolic OH group content was calculated by difference.

The absorbance at 465nm and 665 nm were measured using a Spectronic 20 Genesys Vis spectrophotometer on solutions of 3.0 mg of each HA in 10 mL of 0.05 M NaHCO3. The ratios of absorbance at 465 nm and 665 nm gave the E4/E6 ratio (17).

RESULTS AND DISCUSSION

Charge-pH curves (Q vs. pH) of all humic acids obtained from the potentiometric titration curves (corrected for blank effects) are shown in fig. 1 (a, b, c and d) in absence (I=0 M) and presence of background electrolyte (I=0.1 M). The solid lines are calculated on the basis of sixth degree polynomial equation of the form:

cmol of H+ bound or release/ kg of HA(Q) = a(pH)6 + b(pH)5+...+f(pH) + g


Fig. 1: Charge development curves of humic acids obtained for I=0 M and I=0.1 M NaCl . The solid lines are calculated on the basis of the sixth polynomial equation. (a) Soil HA (HA-S) (R2= 0.995 for I=0 M and R2= 0.995 for I=0.1 M); (b) Compost HA (HA-O6) ( R2= 0.998 for I=0 M and R2= 0.999 for I=0.1 M); (c) Compost HA (HA-O14) (R2= 0.995 for I=0 M and R2= 0.994 for I=0.1 M) and (d) Compost HA (HA-LP) (R2= 0.998 for I=0 M and R2= 0.998 for I=0.1 M).

This equation is used for smoothing purposes in the range of pH 3 to 10, with R2 values exceeding 0.99 in all cases. In other studies, smaller order polynomials gave less acceptable fits and higher order polynomials resulted in no significant improvement (9). By comparing all the titration curves (fig. 1) it can be seen that the shape of the curves was similar.

The charge development of all HA, except in HA-LP, decreases at pH @ 7. This may be due to a conformational change caused by the electrostatic field inside and around the humic macro-ion developed from the acidic group, which are present in different amount in the different AHs (4). The continuous increase of charge development observed for HA-LP above pH 7 can be due to the presence of soil (@ 30-40%) in the composted mixture. The final product of this composting process has about 41% of ash (measured by ignition for 24 h at 5500C), in the HA extracted. This high content of ash (silicate impurities) is also detected in IR spectrum through the strong absorption band observed in the range from 1000 to 500 cm-1 (1). These absorption bands are not detected in the IR spectrum of the others AHs studied in this research (spectrum not shown). We suggest that this ash holds high quantities of OH groups from the silicate impurities titrated at the same time with weak acids groups (hydroxyls and other) of HA. This is in agreement with phenolic OH content determined using the method proposed by Schnitzer y Gupta (16) (Tabla 1).

Table 1. Acidic functional group contents (cmol kg-1) and E4/E6 ratio of humic acids isolated from composted urban waste and soil.

In the range of pH 3 - 7, the charge development of compost HA were smaller than soil HA, except for HA-LP, which was similar to soil HA. This similar behavior of HA-LP could be because the soil in composting mixture could incorporate the organic C derived from decomposition of vegetal materials, especially when this was thoroughly mixed with soil, i.e. the soil organic matter behaved as a sink for C from vegetal residues in forms relatively resistant to further decomposition (18).

In this range of pH (3 ­ 7) the acidic group titrated are, mainly, carboxylic groups. The COOH content of HA-LP are lower than in HA-S (Table 1), probably due to the high ash content in the former. We suggest, that with the conventional method is difficult to quantify it, because the COOH groups are complexed with silicates impurities.

Figure 1 (a - d) clearly shows the effect of ionic strength. For high ionic strength, the screening of the charge is more efficient, which results in lower electrostatic interactions. The lower electrostatic interactions make the surface more easily charged. Therefore, the higher the ionic strength, the lower the pH for which the humic particle has a certain charge. De Wit et al., (7, 8) suggest that as ionic strength increases, the surface potential of a hypothetical humic acid decreases as a function of the pH because of the screening of the charges caused by the salt effect. Thus, the dissociation (or charge development) will be enhanced with increasing salt level. When the charge is better screened by electrolyte ions, the electrostatic effects will impede or hind the dissociation processes.

Figure 1 (a - d) shows that HAs are more negatively charged at high than at low ionic strength at any given pH value, especially in soil HA. This may be due to a more complete and efficient humification process that occurs in natural conditions in soils than that extracted from compost materials. Another reason could be the low ash content (< 4%) for soil HA. The effect of salt level should be more evident in HA with a low ash content. In the high ash sample, the silicate impurities can cause an initial background ionic strength and thus reduce the impact of the salt effect that was to be induced by the addition of the background electrolyte. The negative charge at the lowest pH measured in our samples (pH @3) is already considerable, which suggests that very acidic groups must be present. This is more evident for I = 0.1

M and especially for soil humic acids.

The period of composting influence charge development. The negatively charged sites on HA-O14 are higher than those extracted from compost aged for 6 month (HA-O6) (Fig 1 b and c). This behavior indicates that during the composting process the organic matter, in particular HA increases its cation exchange capacity (CEC). Roig et al., (19) reported that the oxidated groups responsible for the CEC are mainly in humic and fulvic acids. The larger the oxidation and decomposition of the organic matter, the more the production of acidic functional groups, in particular carboxylic functional groups, which increases the CEC.

The first derivatives were obtained on the bases of a smoothing function (sixth degree polynomial equation) through the experimental data for both ionic strength (Fig. 2 a and b). We can separate the acidic functional groups in all of our HAs studied into three site domains: the strongest groups with a pKapp value around 3 assigned to o-COOH, the simple carboxylic groups (COOH) with pKapp around 6, and phenolic groups with pKapp around 9. Each of these are discussed below.


Fig. 2: Apparent affinity distribution and buffer capacity curve obtained from the first derivative through charge-pH curves smoothing with sixth polynomial equation for all the humic acids in the range of pH 3-10. (a) I=0 M and (b) I=0.1M NaCl

The characteristics of all the derivatives obtained for the different samples are similar. Distinct maximum and minima of dQ/dpH are evident between pH 3 and 10 for the humic substances investigated in this study (Fig. 2 a and b). The broad maximum occurring between pH 5 to 7, in absence of background electrolyte (I=0 M), reflects the abundance and distribution of carboxylic functional groups. This maximum occurs at lower pH value and is higher for humic acids extracted from soil (HA-S) than that observed for compost HA. These characteristics indicate that HA extracted from soil has more and stronger carboxylic groups than those present in compost HA, probably due to the different humification processes that take place in soil than in the compost.

Functional groups at the low and high end of the pH range are, therefore, relatively common in all samples. The peaks height observed at the low pH end are higher for HA-LP and HA-S than for HA-O6 and HA-O14, which could be due to the high soil content in the composting mixture in La Para. Thus, these peaks indicate a high content of very acidic sites for both HA (AH-LP and HA-S). The peaks observed at the higher pH end (8.5-10) can be assigned to phenolic and/or other weak acidic functional groups like secondary or even tertiary amines, which can be titrated together with phenols (20).

The well defined peak present in HA-LP at pH @ 9 can be assigned to the contribution of hydroxyl groups supplied by the silicate impurities in addition to the weak acidic functional groups.

The peak observed at pH @ 10 for HA-O14 is higher than that present in HA-O6, probably due to the higher nitrogen content in the former (NAH-O14 6%; NHA-O6 4%) and for the higher ash content present in HA-O14 than in HA-O6 (HA-O14 Ash = 5.4%; HA-O6 Ash = 2.2%).

The maximum around pH 6 observed for compost HA (HA-O6, HA-O14 and HA-LP) are wider and flatter than the maximum of HA-S. This could be indicating that in HA extracted from composted materials the probability to find sites with pKapp values in the range pH + dpH is greater than in HA-S. Thus, the heterogeneity of compost HA is higher than those extracted from soil. This may be due to the different and incomplete humification processes that take place during the relatively short transformation periods in which composting is carried out. The differences in the humification process, are shown through the E4/E6 ratio (Table 1). This ratio can explain, in some sense, the heterogeneity of the HA studied, because it is related to the degree of condensation of the aromatic C network, with a low ratio indicative of a relatively high degree of condensation, and a high value of E4/E6 reflect the presence of relatively large proportion of aliphatic structure (17).

The shape of the apparent affinity distribution is changed as ionic strength increases from I = 0 to I = 0.1 M. (Fig 2 a and b). For HA-LP and for HA-S the maximum are shifted to lower pH values. For HA-O6 the maximum at pH @ 6 is lower than the maximum obtained with I=0 M. For HA-O14 the presence of background electrolyte does not change the curve significantly.

Changes observed in the maximum around pH 6 are more evident for soil HA (HA-S) than for compost HA. This behavior could be connect to the limited and incomplete humification process that take place in composting mixtures.

In general, the maximum around pH 6 is wider and flattener than that observed without background electrolyte (I=0 M) in all of the HA studied, indicating more heterogeneity on the humic macro-ions at the higher ionic strength. Thus, the probability of finding charged sites or binding sites is distributed over a wider pH range as the salt level increases. The higher salt levels are more representative of the actual situation in field soil solutions.

For both ionic strengths, analyzing the first derivative as a buffer capacity distribution of HA, we can see that a broad maximum take place between pH 5 and 7, which is associated with an abundance of carboxylic acid functional groups. These acidic groups are the most important in soil solution because they are completely ionized at soil pH and contribute to the soil buffering capacity.

The buffer capacity of soil HA (HA-S) is greater at both ionic strength (I=0; I=0.1 M) than that observed in compost HA (fig.2 a and b). This highlights the quite different nature of HA extracted from compost materials, which has undergone a short period of biological transformation. Nevertheless, the changes observed in the buffer capacity distributions with salt level mimics the behavior of the soil HA.

The buffer capacities (maximum around pH 6) of HA extracted from compost (HA-O6 and HA-O14) increase as the time of composting increased due to the higher period of maturation. This is also probably due to the increase in oxidation and decomposition of the organic matter, which leads an increase in carboxylic groups content and, thus, the buffering capacity.

The buffer capacity is largest for HA-LP. Once again, this is probably due to the high soil content in the composting mixture, which makes the sample more similar to soil HA.

Increasing the ionic strength will increase and shift the buffer capacity to lower pH values (from pH 5.5 to pH 4.5) for HA-S and from pH 6 to pH 5.5 for HA-LP. It does not produce significant changes in HA-O14. The maximum for HA-O6 is lower at I = 0.1 M than that observed at I=0. Again, the different behavior for the two latter HA (HA-O6, HA-O14) may be due to the different period of composting.

The buffer capacity of HA extracted from compost could contribute to the total soil buffer capacity when the composted material are mixed with agricultural soil (6). Compost HA has a lower buffer capacity than those extracted from soil, especially at natural soil pH values. Nevertheless, compost material may contribute to the total soil buffer capacity when these materials are mixed with agricultural soil. The negative charged sites on compost HA should contribute to the soil cation exchange capacity and thus increase the nutrient availability. We suggest that HA extracted from composted material are macromolecules "like soil humic acids", i.e. "humiclike fraction". Once mixed with agricultural soil, they can continue with the humification process under natural conditions and become closely integrated with the highly stable soil organic matter pool. For these reasons, compost materials used as organic amendments to agricultural soil could improve chemical and physical soil fertility. This is especially useful in soils with heavy losses of organic matter, such as losses due to climatic conditions or intensive cultivation.

CONCLUSIONS

Because of the diversity of HA formation, uniform behavior cannot be expected for this polydisperse product, and only trends can be predicted. Humic acids extracted from compost have some characteristics and behavior similar to those obtained from soil. The potentiometric titration is a simple and suitable method to improve our knowledge of the charge behavior, acid-base behavior and heterogeneity characteristics of HA macromolecules.

The negative charge development of HA extracted from composted material is lower than those extracted from soil and increases as ionic strength increases. The amount of carboxylic groups is lower in compost HA than in soil HA, which indicates a deficient and incomplete humification process. The heterogeneity of HA extracted from compost is higher than those extracted from soil, probably due to the different and limited humification processes that take place when composting solid urban waste compost.

As the time of composting period increases the humification processes that take place trends to produce compost HA with similar characteristics to soil HA. When the composting process is carried out in the presence of a high soil content, the HA in the final product (compost) has more properties closer to the native soil HA. This is presumably due to the extensive incorporation of the native soil HA to form a product that is relatively resistant to further decomposition.

ACKNOWLEDGMENTS

SeCyT - UNC are gratefully acknowledged for financial support.

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