Environ. Sci. Technol. 2005, 39, 7141-7146
Acid-Base Properties of Humic and Fulvic Acids Formed during Composting C EÄ S A R P L A Z A , * , † N I C O L A S E N E S I , ‡ ALFREDO POLO,† AND GENNARO BRUNETTI‡ Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Cientı´ficas, Serrano 115 dpdo., 28006 Madrid, Spain, and Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, University of Bari, Via Amendola 165/A, 70126 Bari, Italy
The soil acid-base buffering capacity and the biological availability, mobilization, and transport of macro- and micronutrients, toxic metal ions, and xenobiotic organic cations in soil are strongly influenced by the acid-base properties of humic substances, of which humic and fulvic acids are the major fractions. For these reasons, the proton binding behavior of the humic acid-like (HA) and fulvic acid-like (FA) fractions contained in a compost are believed to be instrumental in its successful performance in soil. In this work, the acid-base properties of the HAs and FAs isolated from a mixture of the sludge residue obtained from olive oil mill wastewater (OMW) evaporated in an openair pond and tree cuttings (TC) at different stages of composting were investigated by a current potentiometric titration method and the nonideal competitive adsorption (NICA)-Donnan model. The NICA-Donnan model provided an excellent description of the acid-base titration data, and pointed out substantial differences in site density and proton-binding affinity between the HAs and FAs examined. With respect to FAs, HAs were characterized by a smaller content of carboxylic- and phenolic-type groups and their larger affinities for proton binding. Further, HAs featured a greater heterogeneity in carboxylic-type groups than FAs. The composting process increased the content and decreased the proton affinity of carboxylic- and phenolic-type groups of HAs and FAs, and increased the heterogeneity of phenolic-type groups of HAs. As a whole, these effects indicated that the composting process could produce HA and FA fractions with greater cation binding capacities. These results suggest that composting of organic materials improves their agronomic and environmental value by increasing their potential to retain and exchange macro- and micronutrients, and to reduce the bioavailability of organic and inorganic pollutants.
Introduction Soil humic substances (HS) are universally recognized to exert a number of essential physical, chemical, and biological functions to sustain soil fertility and protect soil from * Corresponding author phone: +34-914115301; fax: +34914115301; e-mail:
[email protected]. † Consejo Superior de Investigaciones Cientı ´ficas. ‡ University of Bari. 10.1021/es050613h CCC: $30.25 Published on Web 08/09/2005
2005 American Chemical Society
degradation (1). Humic substances, of which humic acids (HAs) and fulvic acids (FAs) are the major fractions, feature a colloidal, polydispersed, and polyelectrolytic character, a mixed aliphatic and aromatic nature, and the presence of various chemically reactive functional groups, including carboxyls and phenolic and alcoholic hydroxyls, with pHdependent charge properties (1). The acidic functional group contents and proton binding affinities of HS significantly affect the acid-base buffering capacity of soils, and have a marked influence on the speciation of cations in soil solid and liquid phases. Thus, HS can control the biological availability, physicochemical behavior, and environmental fate of macro- and micronutrients, toxic metal ions, and xenobiotic organic cations (e.g., pesticides such as paraquat, diquat, and s-triazines) (1-3). Composting consists of the biological transformation of decomposable organic materials through a controlled oxidation process that results in the release of carbon dioxide, water, and minerals, and the production of biologically mature and stable and chemically complex organic compounds resembling native soil HS. The acid-base properties of these HS-like compounds are believed to be instrumental to obtain a successful agronomic performance of the compost in soil (4-8). Potentiometric titration methods, in which pH is monitored continuously as increments of acid or base are added, combined with thermodynamic models to describe the shapes of the titration curves can provide reliable, detailed, and valuable information about the acid-base properties of HS in terms of site density and proton-binding affinity (9). In particular, the recently developed nonideal competitive adsorption (NICA)-Donnan model can successfully account for the extremely heterogeneous mixture of acidic functional groups involved in proton binding and the various ionic strength effects arising from the polyelectrolytic nature of HAs and FAs of different origin and nature (10-17). To date, however, no NICA-Donnan data are available in the literature for proton binding by HS-like compounds formed during composting. The objectives of this work were to provide a detailed description of the evolution of the acid-base characteristics and to evaluate the ability of the NICA-Donnan model to describe the proton binding behavior of the humic acid-like (HAs) and the fulvic acid-like (FAs) fractions from a mixture of olive oil mill wastewater (OMW) sludge and tree cuttings (TC) at different stages of composting.
Materials and Methods Humic and Fulvic Acid Samples. The HA and FA samples used in this work were isolated according to conventional procedures from a mixture of OMW sludge (58%) and TC (42%) after 13, 56, and 118 days of composting (C13, C56, and C118, respectively). The main chemical properties of the OMW sludge and TC, the composting system, the chemical changes occurring in the OMW sludge-TC mixture during composting, and the isolation procedures and major compositional, structural, and functional properties of HAlike and FA-like fractions are described in detail elsewhere (7, 8). Potentiometric Titrations. Fully automated potentiometric titrations were carried out using a Mettler Toledo (Highstown, NJ) DL77 titrator equipped with a pH electrode Mettler DG-111-SC (Highstown, NJ) that was previously calibrated with standard buffers at pH 4.00, 7.00, and 9.22. The HAs and FAs were prepared for titration by dispersing 50 mg of each freeze-dried sample in 50 mL of deionized VOL. 39, NO. 18, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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distilled water in 100-mL thermostatic vessels. The pH was adjusted to a value of ∼10.2 by addition of KOH, and the mixture was stirred for 1 h under N2 gas. To eliminate hysteretic effects, two successive acid titrations with 0.05 M HNO3 until pH ∼3.3 (backward titration) followed by forward titrations with ∼0.05 M KOH until pH ∼10.2 were conducted. Then three further backward and forward titrations at ionic strengths of 0.05, 0.1, and 0.3 M (adjusted with KNO3) were performed, which were considered for calculations. Acid and base titrants were dispensed using automatic syringes in aliquots of 0.002-0.05 mL in order to have an electrode potential variation smaller than 10 mV. Samples were maintained at a constant stirring speed, at a temperature of 298 K, and under N2 atmosphere throughout the titrations. The volume of titrant added and pH (when stable for 30 s with a drift of no more than (0.02 mV) were recorded after each addition of titrant. Forward and backward titrations gave identical results. In this paper, only backward titrations are reported.
Theoretical Basis Modeling Proton Binding. In the Donnan model, the HA and FA are considered to form an electrically neutral gellike phase having a volume throughout which there is a uniform, averaged electrostatic potential known as the Donnan potential (ψD). The electroneutrality condition of the gel phase is given by
q/VD +
∑ z (c i
D,i
- ci) ) 0
(1) kg-1,
where q is the net charge of the HA in mol VD is the Donnan volume or the specific volume of electrolyte in the gel phase in L kg-1, zi refers to the ionic charge of the ion i (including sign), cD,i is the molar concentration of the ion i in the gel phase, and ci is the molar concentration of the ion i in the bulk solution. According to the Boltzmann distribution law, cD,i can be related to ci by
cD,i ) ci exp(- ziFψD/RT)
(2)
where F is the Faraday’s constant in C mol-1, ψD is the Donnan potential in volts, R is the gas constant in Joule mol-1 K-1, and T is the temperature in K. The exponential term is often called the Boltzmann factor (χD,i). The Donnan model requires that VD is known over the complete range of solution conditions. According to Benedetti et al. (11), VD decreases with increasing ionic strength following the empirical relationship
log VD ) b(1 - log I) - 1
(3)
where I is the ionic strength and b is an empirical parameter related to the size of the molecules and describing how VD varies with ionic strength. When a continuous distribution of binding sites is assumed, the fraction of all sites occupied by the species i (θi) is given by the following integral equation
θi )
∫
∆log Ki
θi,lf(log Ki)d(log Ki)
(4)
where θi,l is the local adsorption isotherm, which describes the binding of i to a group of identical sites, f(log Ki) is the distribution function of the affinity constant Ki, and ∆ log Ki is the range of possible log Ki values. For a multicomponent system, the use of the competitive extended HendersonHasselbalch equation, as the nonideal local isotherm, in combination with the Sips distribution function (quasiGaussian), leads to the basic NICA equation for the specific binding of ions i in the competitive situation (11, 12, 18-20) 7142
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θi )
(K ˜ icD,i)ni
∑ i
(K ˜ icD,i)ni
[ ×
∑(K˜ c
i D,i)
ni p
]
i
1+[
∑
(5) (K ˜ icD,i)ni]p
i
where K ˜ i represents the median affinity constant for the component i, p is the width of the affinity distribution, which reflects the intrinsic chemical heterogeneity of HS, and ni represents the stoichiometry of the binding reaction. The bound amount of ion i, Qi, is given by
Qi ) θiniQmax
(6)
where Qmax is the overall site density, and the stoichiometry factor ni reflects the number of ions i bound to each reference site. When the proton is used as the reference ion the overall site density becomes that of the proton and the bound amount of ion i should be scaled by ni/nH rather than by ni alone:
Qi ) θi(ni/nH)Qmax,H
(7)
The affinity distributions for proton binding by HS typically exhibit two broad peaks, indicating that there are two major types of binding sites (21-23). The NICA model can be extended to reflect a bimodal distribution of proton affinities by
Qi ) θi,1(ni,1/nH,1)Qmax,1 + θi,2(ni,2/nH,2)Qmax,2
(8)
where the suffixes 1 and 2 denote two types of binding sites. For proton binding by HA and FA in the absence of other specific cation binding, the amount of protons specifically bound, QH, is given by
(K ˜ H,1cD,H)m1 (K ˜ H,2cD,H)m2 QH ) Qmax,1 + Qmax,2 (9) m1 1 + (K ˜ H,1cD,H) 1 + (K ˜ H,2cD,H)m2 where m1 and m2 define the widths of the distributions and are a measure of both the ion-specific nonideality and the intrinsic chemical heterogeneity of the HA and FA (mH,j ) nH,j × pj; j ) 1, 2) (12). The total amount of protons bound, QH,t, equals the sum of protons specifically bound and the amount of protons bound electrostatically in the Donnan phase
QH,t ) QH + VD(cD,H - cH)
(10)
The ionic strength and the concentration of protons were calculated for each titration point by an iterative procedure from the concentration of the background electrolyte and pH. Activity coefficients were derived from the calculated ionic strength using the Davies equation (24). The relative net charge on HA and FA at a given proton concentration and ionic strength, which equals the relative amount of protons bound to HA and FA (∆QH), was calculated by subtracting theoretical blank titrations from HA and FA titrations, i.e., by subtracting, for each data point, the amount of titrant required to increase the pH of an equivalent volume of background electrolyte solution. To obtain the absolute amount of protons bound (QH), the value of the initial amount of proton binding (QH0) was added to ∆QH for each point on the titration curve
QH,t ) ∆QH + QH0
(11)
Once the ∆QH, the free proton concentration, and ionic strength were calculated for each titration point, the optimum set of NICA-Donnan model parameters (i.e., b, Qmax,1, Qmax,2,
TABLE 1. Fitting Parameters of the NICA-Donnan Model for Proton Binding by Humic Acid-Like (HA) and Fulvic Acid-Like (FA) Fractions Isolated from the Olive Oil Mill Wastewater Sludge-Tree Cutting Mixture, Compared to the Values Derived by Milne et al. (15) for a Generic HA and FA during Compostinga HA and FA samples
composting time (days)
r2
RMSE
b
Qmax,1
log K˜ H,1
m1
Qmax,2
log K˜ H,2
m2
Qmax,1 + Qmax,2
C13-HA C56-HA C118-HA generic HA C13-FA C56-FA C118-FA generic FA
13 56 118
0.9998 0.9999 0.9989
0.0130 0.0083 0.0345
13 56 118
0.9996 0.9990 0.9984
0.0203 0.0402 0.0476
0.67 0.61 0.46 0.49 0.89 0.66 0.62 0.57
3.08 3.14 3.63 3.15 3.60 4.63 4.68 5.88
4.94 3.47 2.80 2.93 3.23 2.71 2.55 2.34
0.41 0.51 0.43 0.50 0.58 0.58 0.56 0.38
0.40 1.73 1.87 2.55 2.23 2.21 2.52 1.86
8.33 7.55 7.15 8.00 7.22 6.88 6.96 8.60
0.95 0.26 0.20 0.26 0.19 0.24 0.20 0.53
3.48 4.87 5.50 5.70 5.82 6.84 7.20 7.74
a r2, coefficient of determination; RMSE, root-mean-square error after optimization; b, empirical parameter describing how the Donnan volume ˜ H,1, median value of affinity distribution for varies with ionic strength; Qmax,1, carboxyl group content (mol kg-1 on moisture- and ash-free basis); K proton binding by carboxyl groups; m1, width of proton-affinity distribution of carboxyl groups; Qmax,2, phenolic OH group content (mol kg-1 on ˜ H,2, median value of affinity distribution for proton binding by phenolic OH groups; m2, width of proton-affinity moisture- and ash-free basis); K distribution of phenolic OH groups.
K ˜ H,1, K ˜ H,2, m1, and m2) for each HA and FA sample was obtained by iteratively varying the adjustable parameter values (including QH0) until the sum of the squares of the differences between observed and fitted values of ∆QH was minimized. Titration data at pH < 3.5 or pH > 10.0 were considered unreliable, and were not used for calculations. Full, unconstrained optimization was achieved using a hybrid algorithm based on the Marquadt-Levenberg approach. The computer program FIT 2.5 was used for calculations (25).
Results and Discussion Chemical Characteristics of Humic and Fulvic Acids. The compositional, structural, and functional chemical properties of HAs and FAs examined in this paper have been previously described (7, 8). Briefly, at the initial stage of composting, the HA-like fractions from the OMW sludge-TC mixture were characterized by a marked aliphatic character, small O and acidic functional group contents, marked presence of proteinaceous materials and partially modified lignin moieties and polysaccharides-like structures, extended molecular heterogeneity, small organic free radical contents, and small degrees of aromatic ring polycondensation, polymerization, and humification. With increasing the composting time, a loss of aliphatic materials and carbohydrates, and an increase of oxygenation, acidic functional groups, S- and N-containing groups, and aromaticity occurred in the HA-like fraction. Compared with the corresponding HA-like fractions, the FA-like fractions featured a different elemental and acidic functional group composition, larger E4/E6 ratios, much larger relative fluorescence intensity values, and markedly different Fourier transform infrared (FTIR) and fluorescence spectra. With increasing the composting time, the trends of the elemental composition (except for S content) and acidic functional group content of the various FAs were similar to those of the HAs. Further, FTIR and fluorescence results suggested that a loss of aliphatic materials and carbohydrates and an increase of N-containing groups, aromaticity, and humification degree occurred in both HA-like and FA-like fractions during composting of the OMW sludge-TC mixture. Although the transformations of both fractions seemed to follow quite similar patterns, changes in chemical properties and molecular structure were considerably more evident for the HAs than for the FAs. Charge Versus Proton Concentration Curves. Figure 1 shows the experimental data for proton binding by HAs and FAs isolated from the OMW sludge-tree cutting mixture sampled during the composting process expressed as the negative charge on HA and FA as a function of proton concentration and ionic strength. In agreement with results obtained by other authors on HS of different origins (11,
13-15, 20, 26), the negative charge on the HAs and FAs examined here increases with increasing pH and ionic strength. Also in agreement with previous findings (9, 15, 23), the HAs exhibit smaller charge densities across the entire range of proton concentration and ionic strength than the corresponding FAs. The titration curves for each ionic strength tend to diverge slightly with increasing the composting time, which indicates that the ionic strength dependence of the charge developed by HAs and FAs increase markedly during composting. Further, at any given pH and ionic strength, the charge densities of FAs, and especially HAs, increase as the composting time increases. The different charging behavior of the HAs and FAs examined can be feasibly ascribed to the different contents and proton-binding affinities of their carboxyl and phenolic OH groups, whose dissociation is generally considered to be mainly responsible for the negative charges of HS (1, 27, 28). NICA-Donnan Parameters. Table 1 shows the best-fit parameters for the best-fit lines (Figure 1) generated from the analysis of experimental data using the NICA-Donnan model for a bimodal distribution of proton binding sites of each HA and FA. Consistently with the conventional interpretation of acid-base properties of HS (1, 28, 29), these include: the parameter b, total acidity (Qmax,1 + Qmax,2), carboxyl and phenolic OH group contents (Qmax,1 and Qmax,2, respectively), median values of affinity distributions for proton binding by carboxyl groups and phenolic OH groups (logK ˜ H,1 and logK ˜ H,2, respectively), and the width of protonaffinity distributions of carboxyl and phenolic OH groups (m1 and m2, respectively). The suffixes 1 and 2 denote the two classes of acidic functional groups that are assumed to be, respectively, carboxylic-type groups and phenolic-type groups. The proton-affinity distributions associated with the NICA-Donnan descriptions (Figure 2) were calculated from the first derivative of the curve-model-derived values of charge versus proton concentration in the Donnan phase by applying the condensation approximation method (30). In agreement with results of previous works on similar systems (10-17), the large values of the determination coefficient (r2) and the small residual mean square errors (RMSE) indicate that the NICA-Donnan model fits very well to the experimental datasets for the HAs and FAs examined. However, the considerable overlap of affinity distributions of carboxylic- and phenolic-type groups (Figure 2) suggests an implicit inaccuracy in the NICA-Donnan parameter values discussed comparatively in the following text, particularly for values of phenolic-type groups. The uncertainty in resolving the affinity distributions of carboxylic and phenolic type groups could be minimized by deriving some VOL. 39, NO. 18, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Negative charge of humic acid-like (HA) and fulvic acid-like (FA) fractions isolated from the olive oil mill wastewater sludgetree cutting mixture after 13, 56, and 118 days of composting (C13, C56, and C118, respectively) as a function of proton concentration and ionic strength (I). Symbols represent experimental data obtained from potentiometric titrations (only half the data are shown for clarity), and continuous lines represent fits with the NICA-Donnan model. model parameters from chemical characterization results, e.g., by determining phenolic OH group contents by solid state 13C NMR spectroscopy (14). Parameter b. In agreement with results recently reported by Milne et al. (15) for generic HAs and FAs, the b values of HAs from the OMW sludge-TC mixture are smaller than those of the corresponding FAs. With increasing the composting time, the b values of HAs and FAs decrease markedly. According to Christl and Kretzschmar (14), the parameter b is correlated negatively with particle size and molecular weight. The b value decrease might thus suggest that composting induces an increase of the contribution of high7144
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molecular-weight particles in HAs and FAs. This result is consistent with previously reported gel-chromatographic analysis of molecular weight distribution of HAs isolated from similar systems (31). Acidic Functional Group Contents. The total acidity and carboxyl and phenolic OH group contents (Qmax,1 + Qmax,2, Qmax,1, and Qmax,2, respectively, in Table 1) of HAs are smaller than those of FAs. The carboxyl group contents fall into the ranges reported by Milne et al. (15) for a large number of HAs and FAs, and constitute from 64% to 89% of the total acidity of HAs, and from 62% to 68% for FAs. These results indicate that most acidity of HAs and FAs is attributable to carboxyl
TABLE 2. Acidic Functional Group Contents of Humic Acid-Like (HA) and Fulvic Acid-Like (FA) Fractions Isolated from the Olive Oil Mill Wastewater Sludge-Tree Cutting Mixture during Composting as Measured by Indirect Titrations Using the Ba(OH)2 and Ca(CH3COO)2 Methods, and Correlation Coefficients (r) with Values Estimated by using the NICA-Donnan Model HA and FA samples
composting time (days)
total acidity (mol kg-1)a
COOH (mol kg-1)
phenolic OH (mol kg-1)
C13-HA C56-HA C118-HA C13-FA C56-FA C118-FA r
13 56 118 13 56 118
4.67 6.10 6.26 6.62 6.66 7.48 0.9543b
2.67 3.09 3.21 5.04 4.57 5.26 0.7787
2.00 3.01 3.05 1.58 2.09 2.22 -0.0168
a
FIGURE 2. Affinity distributions derived from the NICA-Donnan parameter values for proton binding by humic acid-like (HA) and fulvic acid-like (FA) fractions isolated from the olive oil mill wastewater sludge-tree cutting mixture after 13, 56, and 118 days of composting (C13, C56, and C118, respectively). groups. Further, the total acidity and carboxyl and phenolic OH group contents of HAs and FAs tend to increase as composting time increases. The acidic functional group contents of HAs and FAs isolated from the OMW sludge-TC mixture previously measured by indirect titrations using the Ba(OH)2 and Ca(CH3COO)2 methods (7, 8) are listed in Table 2. The values of total acidity and carboxyl and phenolic OH group contents obtained by indirect titration methods are generally slightly larger than the corresponding values obtained by using the NICA-Donnan model (Table 1). Further, neither carboxyl nor phenolic OH group contents obtained by indirect titrations are correlated significantly (P > 0.05) with the values estimated by the NICA-Donnan model. According to Ritchie and Perdue (9), it is likely that acidgenerating side reactions may occur when total acidity is measured after a 24-h exposure of HS to extremely alkaline conditions, and/or that part of HS may not be removed as insoluble Ba-salt during filtration. Further, the carboxyl group contents obtained by Plaza et al. (7, 8) might be affected by ion exchange processes possibly occurring during the Ca(CH3COO)2 procedure, which implies that acidic OH groups of HAs and FAs can ionize below the pH of the acetate reaction mixture (1, 28). Further, as in the case of total acidity measurements by the Ba(OH)2 method, incomplete removal of HS during filtration might have occurred.
On moisture- and ash-free basis.
b
P < 0.01.
Proton-Binding Affinities. The median values of affinity distributions for proton binding by carboxylic- and phenolic˜ H,2, respectively) derived for type groups (logK ˜ H,1 and logK HAs and FAs from the OMW-TC mixture correspond very well to values commonly reported for HAs and FAs (10-15, 32), and closely resemble proton affinity constants of monoand polycarboxylic acids and phenols (13, 14). This result supports the statement that proton binding to HAs and FAs is mainly determined by dissociation of carboxyl and phenolic OH groups. The median proton affinity constants of carboxylic- and phenolic-type groups are larger for HAs than for FAs, which indicates that carboxyl and phenolic OH groups of HAs are less acidic than those of the FAs. Further, the logK ˜ H,1 and logK ˜ H,2 values of HAs and FAs decrease with composting time, which implies that the composting process causes an increase of acidity due to carboxylic- and phenolic-type groups of both fractions in the OMW sludge-TC mixture, especially that of the HA-like fraction. Width of the Affinity Distributions. Except for the C13-HA sample, the values of m1 of all HAs and FAs examined are much larger than the values of m2, which is in agreement with previous findings (10-12, 14, 32). As m is considered to reflect the apparent heterogeneity of the site type (11), these results indicate that the low affinity proton binding sites (carboxylic-type groups) exhibit a smaller apparent heterogeneity than the high affinity sites (phenolic-type groups). The m1 values of HAs are smaller than the corresponding values of FAs. In contrast, the m2 values of HAs are similar to those of the corresponding FAs with the exception of sample C13-HA, which features a much greater homogeneity in phenolic OH groups than its FA counterpart. The composting process induces a decrease of the m2 value of HAs, whereas it appears not to substantially affect the heterogeneity of acidic functional groups of FAs.
Acknowledgments This research was supported by the European CommissionCRAFT Contract SOLARDIST-Project N.EVK1-CT-2002-30028.
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(20)
(21)
(22)
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(24) (25)
(26)
(27)
(28)
(29)
(30)
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Received for review March 30, 2005. Revised manuscript received June 30, 2005. Accepted July 11, 2005. ES050613H
