Chemicals from Wastes: Compost-Derived Humic Acid-like Matter as

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Environ. Sci. Technol. 2006, 40, 1686-1692

Chemicals from Wastes: Compost-Derived Humic Acid-like Matter as Surfactant PIERLUIGI QUAGLIOTTO,† E N Z O M O N T O N E R I , * ,† FULVIA TAMBONE,‡ FABRIZIO ADANI,‡ ROBERTO GOBETTO,§ AND GUIDO VISCARDI† Dipartimento di Chimica Generale ed Organica Applicata e Dipartimento di Chimica I.F.M., Universita` di Torino, 10125 Torino, Italy, and Dipartimento di Produzione Vegetale, Universita` di Milano, 20133 Milano, Italy

Compost humic acid-like (cHAL) polymeric matter (MW ) 15610), isolated in 12% yield from food and green waste compost, exhibits very good surfactant properties in aqueous solution: i.e., critical micelle concentration (cmc) ) 403 mg/L and surface tension at cmc ) 36.1 mN/m. Values of cmc are confirmed also by conductivity and phenanthrene solubility measurements. These results, compared with those for other major commercial and research surfactants, propose cHAL as a competitive low-cost biosurfactant.

Introduction Humic acids (HAs) are constituents of soil (1) and natural waters (2) organic matter. Structural information (3) allows forecasting good properties for these compounds as complexing agents, ion exchangers, and surfactants. Most of their role in the natural environment is connected to these properties. There is plenty of literature on HAs. As humic substances constitute the main part of organic matter in soil and natural waters, most of the published work is focused on the interaction of HAs with organic contaminants in soil and water in order to assess the impact and fate of anthropogenic wastes in the environment. A minor part of the literature is dedicated to the study of surface tension properties. Yet the capacity of a chemical compound to lower the water surface tension is a basilar property for envisioning performance in many technological applications (4). In the context of the modern issue of renewable versus nonrenewable sources of chemical compounds for the above uses, we have become convinced that HAs may offer intriguing perspectives. There are very few published papers that consider HAs from this point of view. A most recent work (5) indicates for the first time that a natural nontoxic surfactant such as HA isolated from a Leonardite is capable of removing similar amounts of contaminants from a polluted soil as synthetic surfactants. By virtue of its high HA content, Leonardites are certainly an interesting source of materials for the chemical industry. Humic acid-like (HAL) material however may be obtained in good yields also from composted * Corresponding author phone: +39-333-3500522; fax: +39-0116707591; e-mail: [email protected]. Dipartimento di Chimica Generale ed Organica Applicata, Universita` di Torino, C. M. D’Azeglio 48, 10125 Torino, Italy. † Dipartimento di Chimica Generale ed Organica Applicata, and Centro di Eccellenza NIS, Universita` di Torino. ‡ Universita ` di Milano. § Dipartimento di Chimica I.F.M., Universita ` di Torino. 1686

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agro-industrial and municipal wastes (6). These compost biomasses, so far, have been considered only for soilamending purposes. To stimulate interest in compost HAL for other applications, we report hereinafter the chemical characterization and the surface tension properties of a humic acid-like matter (cHAL) isolated from a composted mix of municipal solid and lignocellulose wastes. To our knowledge, no work has been published so far on the surface tension properties of compost HAL. We are aware however of one paper reporting surface tension data for a compost extract (7), and of a few other papers published on the characterization of surface activity of HAs from a commercial source (8) and isolated from peat (9), soil and sediment (10-12), and brown coal (13). Most of the work on the surface activity of HAs reports that the surface tension of water decreases as the HA concentration increases until at some point it becomes constant despite the addition of more HA. This point is called the critical micellar concentration (cmc), indicating the formation of aggregates. There has been however some dispute in the literature on the meaning of these aggregates, i.e., whether an aggregation phenomenon indicates formation of micelles or not. To solve this issue, Guetzloff and Rice (8) have performed both surface tension and solubility measurements for commercial Aldrich HA in water solution at variable concentrations, using DDT as a hydrophobic probe to demonstrate enhanced solubility capacity by aqueous HA and thus to prove surfactant micelles formation. These authors found at 7.4 g/L HA concentration the coincidence of the change of the slope in both water surface tension and DDT solubility plots versus HA concentration. Such evidence is taken by the authors has definitive, unambiguous proof of the formation of HA micelles at the aggregation point indicated by the surface tension measurements. The use of solubility probes alone for determining the cmc of HAs may be questioned. Hydrophobic probes tend to interact with the hydrophobic moiety of the single HA molecule (14, 15) and the cmc values for a specific surfactant have been found to depend on the nature of the solubility probe (16-18). In contrast, conductivity (19) and surface tension measurements (17) are generally considered the most appropriate techniques for surfactant characterization since they do not rely on the use of molecular probes that could perturb the micelles under investigation. Aside from theoretical dispute, solubility enhancement of hydrophobic compounds in water remains a typical desirable property of surfactants for practical purposes and probe solubility measurements are of great importance as they may provide strong hints of possible fields of application. In the case of the present work, where the potential technological applicability of compost extracts is a major point, we have performed, together with surface tension and conductivity measurements of aqueous cHAL, solubility measurements of phenanthrene (PHE) in the same medium. Phenanthrene was chosen because it is a typical hydrophobic compound, an important environmental contaminant and the object of current soil decontamination studies (5). Phenanthrene solubility studies are therefore particularly important in the present work inasmuch as they may address the specific technological applicability of the above cHAL material to soil remediation.

Experimental Procedures Composting Process. Ten kilograms of a 1:1 v/v mixture of food residues and public parks green wastes were composted for 15 days as previously described (6). The wastes were 10.1021/es051637r CCC: $33.50

 2006 American Chemical Society Published on Web 01/26/2006

TABLE 1. Dataa Found,b-g and Calculatedh for cHaL,b for the Proposed Molecular Structureh in Figure 1, and for Native Ligninc and HAd-f or HALg Isolated from Various Sources empirical formula C w/w % 59.9b 62.7h 59.6-63.8c 50.4-58.8d,e 55.0-61.4f 55.6g

C

H

10 10 10 10 10 10

13.4 13.0 11.8-14.5 7.3-12.7 6.2-9.3 13.1

N

O

0.25-0.52 0.11-0.52 0.97

3.4 2.9 3.3-4.3 4.6-5.8 4.2-5.0 4.3

0.86 0.89

S 0.036 0.052 0.055-0.012

pH

COOH meq/g

ArOH meq/g

MW

MN

MW/MN

4.00

1.10 0.93 0.12-1.44 2.76e 4.8-5.5 2.24

1.90 1.86 1.4-2.2 3.28e 2.0-4.3 2.73

15610 1072

897

17.4

17000e

a C content (w/w %) in ash-free dry matter, elements atoms in empirical formula, pH of a water suspension (12 mg/30 mL) containing 0.01 N NaCl, free carboxylic (COOH) and phenol (ArOH) groups, weight (MW), and number-(MN) averaged molecular weights defined as previously reported (22). b Isolated from compost in this work. c Lignin in wood, wheat straw, rice, and bagasse (23). d Peat (23). e Peat (9 ). f Soil (11). g Seven months city refuse compost (24). h Proposed structure in Figure 1.

TABLE 2. Assignments and Relative Area of 1H NMR Bands, and H Distribution in Proposed Molecular Structure in Figure 1

assignment

H in aliphatic C for hydrocarbon chains substituted at β or farther C, or in CH2 and CH3 bonded to aromatic C, or to carboxylic or amide groups

H in methine group bonded to aromatic C, or in aliphatic C bonded to O or to N

aromatic and olefinic H

band δ range (ppm) band relative area (%) H distribution (%) in Figure 1

1.0-2.5 75.30 78.21

2.5-4.2 15.15 12.82

5.5-8.0 9.55 8.97

TABLE 3. Assignments and Relative Area of 13C CPMAS NMR Bands and C Distribution in Proposed Molecular Structure in Figure 1

assignment

aliphatic C bonded to other aliphatic chain or to H total phenol or short long aliphatic O-CH3 or O-alkyl di-O-alkyl aromatic phenyl ether carboxyl chain chain C N-alkyl C C C C C C

band δ range (ppm) 0-32 band relative area (%) 32.3 C distribution (%) in Figure 1 36.8

32-53 12.9 12.3

0-53 45.2 49.1

53-63 8.4 8.8

obtained from a separate source collection facility located in La Spezia, Italy. The starting mix and the final compost product were characterized as previously reported (20) by the data reported in the Supporting Information file. Isolation of cHAL. The compost was treated for 24 h at 65 °C under N2 with aqueous 0.1 mol L-1 NaOH and 0.1 mol L-1 Na4P2O7 at a 1:50 w/v compost/solution ratio (21). The resulting suspension was cooled to room temperature and centrifuged at 6000 rpm for 20 min. The supernatant solution was separated. The solid residue was washed repeatedly with distilled water until the supernatant liquid phase was clear. All collected liquid fractions were mixed and acidified with 50% sulfuric acid to pH < 1.5. The precipitated cHAL fraction was separated by centrifugation as above, washed with water until the final washing had neutral pH, vacuum-dried at 60 °C, and weighed. The extraction yield for the final cHAL product was 12% of compost dry matter. Chemical Characterization of cHAL. Chemical characterization data are reported in Table 1. The 1H NMR spectrum for cHAL in 3 M NaOD D2O solution, shown in the Supporting Information file and summarized in Table 2 hereinafter, was recorded on a JEOL EX 400 (B0 ) 9.4 T, 1H operating frequency 399.78 MHz). The solid-state 13C NMR spectra (see Supporting Information and Table 3 hereinafter) were acquired at 67.9 MHz on a JEOL GSE 270 spectrometer equipped with a Doty probe. The cross-polarization magic angle spinning (CPMAS) technique was employed and for each spectrum about 104 free induction decays were accumulated. The pulse repetition

63-95 95-11 0 110-140 140-160 9.2 3.5 14.8 6.8 7.0 3.5 15.8 5.3

keto C

160-185 185-215 11.5 0.6 10.5

rate was set at 0.5 s and the contact time at 1 ms, the sweep width was 35 kHz, and MAS was performed at 5 kHz. Under these conditions, the NMR technique provides quantitative integration values in the different spectral regions (25). Chemical shifts of NMR resonances are referred to tetramethylsilane. Missing experimental details are reported in the Supporting Information. Other Measurements. Room temperature data from surface tension (γ, mN/m), specific conductivity (κ), and PHE solubility (S) measurements in aqueous solutions at pH 7 containing cHAL at variable concentrations (Cw ) 0.005-1 g/L) are reported in Figure 2. Experimental details for these measurements were in accordance with previous work (9, 26-28) and are reported in the Supporting Information.

Results and Discussion Chemical Characterization of cHAL. For the assessment of the nature of cHAL used in the present work, we have used the data in Tables 1-3 and the results of IR spectra. All spectral assignments reported hereinafter are in accordance with previous work on humic-like matter (23). The C content for cHAL in Table 1 falls within the range of values for lignin in several wood and other vegetable sources and for HAs and HAL matter isolated from several soil and compost sources. For these materials, empirical formulas from micro-analytical data are usually calculated based on 10 C atoms (methoxy)phenyl-propyl (MeOPhPr) repeating unit, as this unit is agreed to represent the parent lignin in vegetable species (1). The VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Proposed molecular fragment for cHAL. H bonded to C omitted; sinusoidal bold lines indicate other fragments C, O, N atoms. data in Table 1 show that HA isolated from peat and soil and HAL from city refuse compost are characterized by higher O/C ratio and higher total acidity than native lignin does, and in addition by the content of N which is absent in native lignin. These features depend on the material source and on the conditions and duration of the biodegradation process. Compared to the above HAs and to HAL isolated from city refuse composted for several months, our cHAL sample isolated from a compost at shorter biodegradation time (15 days) has the same N/C ratio but significantly lower O/C ratio, and consequently lower content of carboxylic and phenol acid groups. For cHAL, the NMR spectroscopy data (Tables 2 and 3) clearly indicate a large excess of aliphatic C and aliphatic H over the amount expected for the above MeOPhPr repeating unit, i.e., the total aliphatic C/total aromatic C resonance band area ratio in Table 3 results of 3.05 versus 0.6 expected for MeOPhPr; the total area for the 1H resonance bands assigned to aliphatic H in CH3, CH2, and CH in Table 2 yields a total aliphatic H content of 90.5% versus a maximum of 71.4% for unsubstituted MeOPhPr. Another important feature of the data in Table 3 is the substantial content of carboxyl and phenol or phenyl ether C. Indeed, during composting of municipal (24), animal, and/or agricultural wastes (21), polysaccharides and fats are easily mineralized, whereas lignin matter is significantly more recalcitrant toward microbial degradation. Native lignin in this process is altered by demethoxylation, oxidation to carboxylic acid, and nitrogen fixation. For cHAL the presence of an OPhPr moiety is supported by both the 13C resonance in the 53-63 ppm range (Table 3) and the 1H resonance at 2.5-4.2 ppm (Table 2). In the spectra of humic-like materials, these resonances indicate the presence of p-hydroxyphenyl, syringyl, and/or guaiacyl moieties bonded to the alkyl C chain. Also, the IR spectrum (see Supporting Information) allowed assessment of the presence of several functional groups and C bonds from peak absorbances. These were found at 1718 cm-1 for free carboxylic acid groups, at 1518 cm-1 for amide groups, at 1458 cm-1 for aromatic rings, at 1278 cm-1 for C-O bonds in phenol, phenyl ether, and COOH groups, in the 20003700 cm-1 range for O-H and/or N-H groups in COOH, phenol, and/or amide groups, and at 2928 and 2855 cm-1 for protonated aliphatic C chains (23). The potentiometer titration curve (see Supporting Information) exhibited two inflection points at pH 8.42 and 4.80. These, based on the spectroscopic data and on the order of decreasing pH, were interpreted as the end points of the phenoxide and carboxylate functional groups titration, respectively, and allowed calculation of the distribution of total acidity reported in Table 1. A plausible molecular fragment in polymeric cHAL is shown in Figure 1. Such a model is based on the lignin 1688

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monomer concept of Flag (1), i.e., using an oxyphenylpropyl (OPhPr) unit as a building block. Many other similar theories (1) have been published on molecular structures of humic substances. As these substances are commonly considered to be broken down from the large plant biopolymers, all theories are based on aromatic constituents or phenols obtained by a variety of degradation analyses. As plant decay materials, as well as our starting compost waste mix, contain variable concentrations of polysaccharides, protein, and lipids in addition to lignin, incorporation of carbohydrate, protein, and lipid structure memories in the various published structures of humic substances has been taken into account depending on the source of humic material and on analytical data. The proposed structure in Figure 1 was built by considering all found aromatic C in Table 3 as part of the above OPhPr block and then accommodating the found residual aliphatic C and functional groups in order for the final structure to fit the experimental data. The validity of this model may be appreciated from Tables 1-3 showing the vis-a`-vis comparison of its elements and functional groups composition with the experimental data for cHAL. Its structure may be visualized as an elongated hydrophobic portion made by a central 9 C long chain and by peripheral 2-3 C short chain aliphatic amide and di-O alkyl ether groups, to which two polar hydrophilic substituted propylphenol hydrocarbon residues are attached. In this structure the memory of the main molecular constituents of the used compost waste mix may be found as molecular fragments left over by the biodegradation of polysaccharides, protein, lipids, and lignin. Virtually, 14.5 mol of the above molecular fragment should be bonded to one another to reach the experimental MW value in Table 1. However, as cHAL is not a synthetic polymer, the fragment in Figure 1 cannot be taken as the only repeating unit in the polymer. In reality, to account for the biological origin and the wide molecular weight distribution (MW/MN ) 17.4 in Table 1), a range of different molecular fragments and molecules with greater and lower molecular weight is more likely to compose cHAL. We are aware that other structures could be drawn based on the experimental data. For instance, one could start by considering all or part of the aromatic C assembled to form fused aromatic rings and then accommodate in various ways the residual aliphatic C and functional groups. The analytical tools used in this work do not allow assessment of the presence of fused rings. At this scope Py-GC/MS studies are more indicative. Fused aromatic rings however are more likely to be present in fossil material than in short time biodegraded wastes like the compost of this work. In any case, with this limitation in mind, it is shown hereinafter that the proposed model for cHAL accounts satisfactorily for the collected surface tension and solubility data. In addition, we feel that the conversion of a set of numbers as in Tables 1-3 into a graphical, although virtual, representation like Figure 1 helps the reader to have a longer memory of the nature of the investigated material. Surfactant Properties. The results of the surface tension measurements (γ), conductivity (κ), and PHE water solubility (S) versus cHAL concentration in water (Cw, mg/L) are all shown in Figure 2. For each type of measurement the plot shows a significant change of slope occurring roughly at the same Cw value in all cases. We define this Cw value as the surfactant critical micellar concentration (cmc). In principle, surfactants should give a plot where two clear linear regimes, premicellar and postmicellar, can be evidenced. This is mostly found for simple surfactant molecules with one polar head. Oligomeric surfactants, such as the gemini surfactants (26, 29, 30), show a gradual transition between the two regimes whose extension may be very large. These surfactants are made of molecules in which two or more polar groups are connected by lipophilic chains of variable length. Their

FIGURE 2. Surface tension (b), conductivity (2), and PHE solubility (0)-log cHAL concentration (Cw) plot. oligomeric or polymeric nature is a key point in determining their capacity to micellize (31). The same could be expected for polymeric HAL matter. When these materials are in solution as single molecules, they lie at the air-water interface to expose the lowest possible hydrophobic surface to water. At higher concentration, when the air-water interface is saturated, the excess surfactant molecules aggregate forming micelles in the bulk water phase. In this form several molecules are held together by intermolecular forces to yield spherical or pseudo-spherical clusters where hydrophobic surfaces stay in the inner micellar core and polar heads are directed toward the water phase. In a typical case, the premicellar and the postmicellar regimes are defined by two linear γ-Cw or κ-Cw plots with different slopes. The intersection of the two lines gives the cmc (32-35). However, in cases where a very gradual transition between the two regimes occurs, it is difficult to identify exactly the plot tract for each regime. Reliable cmc values are best achieved by using more than one technique as shown hereinafter. The trend of the γ-Log Cw plot in Figure 2 is typical of most surfactants, showing a steep slope at low Cw values (the premicellar regime) and tapering off at higher Cw values (the postmicellar regime). The coordinates values at the slope change point allow the appraisal of the two major surfactant parameters, i.e., the critical micelle concentration (cmc) and the corresponding surface tension (γcmc). From Figure 2 these values are estimated as 403 mg/L and 36.1 mN/m, respectively. Other parameters of technological importance can be evaluated from the γ-Log Cw plot in Figure 2 as reported in the Supporting Information. The κ-Cw data have been collected and treated according to a recent paper (35). The κ-Cw plot (see Figure 5S in Supporting Information) shows that κ increases upon increasing Cw and then, at high Cw values, grows with a smaller slope. This trend cannot be readily appreciated in the κ-Log Cw in Figure 2, which due to the effect of the compressed abscissa values shows an apparent slope increase at high Cw. The κ increase in the premicellar regime (at low Cw) is due to the increase of the concentration of the dissolved surfactant, which remains fully ionized. When the surfactant concentration is high enough, micellization occurs, giving aggregates that firmly bind some counterions to counteract the coulomb headgroup repulsion. At this point the slope of the κ-Cw plot decreases for two reasons, i.e., the micelles diffusion, and consequently the charge transport, is slower than the diffusion of surfactant monomers, and the ionization degree of the micelles is less than that of the monomers. For the above type of κ-Cw plot, the use of a nonlinear fitting

method which takes into account all points has been proven (26, 34) to allow calculation of the surfactant cmc with a good degree of confidence. This method is based on the observation that a system characterized by a transition between two linear regimes can be described by an analytical function obtained by integration of a sigmoid function. The detailed description of the method can be found in the original paper (34) and is not reported here. For the present cHAL case, the above nonlinear fitting method applied to the κ-Cw plot has given cmc ) 437 mg/L, which is in excellent agreement with the above 403 mg/L value obtained from γ-Log Cw data. In addition to the determination of cmc, the conductivity data allow calculation of (31) the degree of micellar ionization as reported in the Supporting Information. Further support of the above cmc values comes from the PHE solubility (S) measurements. The S-log Cw plot (Figure 2) also shows two linear regimes, premicellar and postmicellar, which can be identified in the two 100-300 and 400-700 mg/L Cw ranges, respectively. It is also evident that the S-Log Cw plot in Figure 2 contains a transition region between the two regimes, and this falls in proximity of the cmc value determined by the γ and κ measurements. According to the literature (18), the equilibrium apparent concentration of PHE (S) in the aqueous surfactant solution can be expressed as

S ) So(1 + Ksub-micCsub-mic + KmicCmic)

(1)

where S and So are the solubility of the solute in the surfactant solution and in pure water, respectively, Csub-mic and Cmic are the surfactant concentration below and above the cmc, and Ksub-mic and Kmic are the distribution coefficients of PHE between surfactant and water below and above the cmc. The linear regression analysis of the experimental S and Cw values in the 100-300 and 400-700 mg/L surfactant concentration ranges yields the following equations and statistical parameters as correlation coefficient (F) and standard deviations for the intercept (si) and slope (ss),

S ) 0.25 + 0.0059Cw

(2)

for the premicellar regime, with F ) 0.989, si ) 0.18, and ss ) 0.0009, and

S ) -5.13 + 0.020Cw

(3)

for the postmicellar regime, with F ) 0.993, si ) 0.92, and ss ) 0.002. Equations 2 and 3 yield the same S values for Cw ) VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 4. Performance Parameters for cHAL Isolated in This Work, for HAs from Various Sourcesand for Other Commercial and Research Surfactants surfactant MP-SALc (36) MKLd (37, 38) Borregaard Lignotech SLe (39) Aldrich SLf (40) SDBSg (18) SDSh (17, 41) CTABi (17) soil HA at pH 7 (11) peat HA (10) peat HA at pH 7 (9) brown coal HA (13) at pH 7 Aldrich HA (8) sedimentary HA (12) at pH 8 compost extract (7) cHAL cHAL cHAL

γcmca (mN/m)

cmcb (%, w/v)

45 37-28 40 67.8

0.30 0.26-0.41 0.63-0.76 10 0.10-0.20 0.23 0.033 2.8j 0.1 0.25 5.0j 0.74 0.07-11.4 1.7j 0.040 0.044k 0.038l

39.5 36.5 52j 55 47.5 50j 50 36.8-46.6 65j 36.1

cmcb (M)

10-4 1.9 × 10-3 (2.9-5.7) × 10-3 8.2 × 10-3 9 × 10-4 1.5 × 10-4 (0.2-1140) × 10-5 2.6 × 10-5 2.8 × 10-5 2.4 × 10-5

a Surface tension at critical surfactant micelle concentration. b Surfactant critical micelle concentration. c Product of the Mannich reaction of phenolized sulfuric acid lignin. d Modified Kraft lignin. e Sulfite liquor lignin: MW ) (63-76) × 103, S % ) 6.2. f Sulfite liquor lignin: MW ) 52000, S % ) 5.2. g Sodium dodecylbenzene sulfonate. h Sodium dodecyl sulfate; cmc by PHE solubility measurements. i Cetyltrimethylammonium bromide; literature (17) and authors’ data. j Lowest γ and highest concentration in the investigated range; no cmc was identified. k From conductivity data in Figure 2. l From PHE solubility data in Figure 2.

382 mg/L. This value should represent the cmc of the surfactant based on PHE solubility measurements and is in good agreement with the values of 403 and 437 mg/L determined by the γ and κ measurements. The comparison of the slope values for the premicellar (eq 2) and postmicellar regimes (eq 3) indicates the enhancement of the distribution coefficient in the postmicellar surfactant concentration range by a factor of over 3 relative to the value in the premicellar concentration range. The PHE solubility values above the cHAL cmc value (Cw ) 0.4-0.7 g/L) range from 5 to 9 mg/L. These values fall in the range (1.78-14.2 mg/L) reported (18) for PHE solubility in aqueous sodium dodecylbenzene sulfonate (SDBS) at much higher surfactant concentration (2.09-8.35 g/L). Indeed, as shown in Table 4, the cmc value for the commercial SDBS surfactant is higher by a factor of 2.5-5 compared to that for cHAL. The proposed representation of the cHAL molecular fragment (Figure 1) allows one to expect a macromolecule that should have the properties of a polyelectrolyte. von Wandruska (3) reports that naturally occurring humic acid molecules exist in coil conformation. This is possible by the chain length and flexibility of the polymeric molecule, which allows folding and coiling in a manner that directs hydrophilic groups toward the water phase and hydrophobic moieties away from it. For these molecules, values of the molecular radius as a function of the molecular weight have also been reported (1). One could envision a similar situation for the cHAL molecules of this work. Based on the molecular fragment represented in Figure 1, these molecules should contain rather long flexible aliphatic C atom chains which can easily fold and coil. The area effectively occupied by each cHAL molecule at the air-water surface is hard to determine because of the polydispersity of the sample molecular weight and of the distribution of charged sites. Nevertheless, estimates of the minimum surface area (Amin) per surfactant molecule which is occupied at the air-water interface can be attempted (17) from surface tension data by use of the equations

Γmax ) -

Amin ) 1690

9

[

]

∂γ 1 2.303nRT ∂ log Cw 1016 ΓNA

(4)

T

(5)

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where Γ (mol cm-2) is the surfactant surface excess concentration (i.e., the concentration of surfactant at the airwater surface), R is the gas constant, T (K) is the absolute temperature, δγ/δ log Cw is the slope of the γ vs Cw plot, NA stands for the Avogadro number, and n is the Gibbs prefactor. This last parameter is equal to the number of chemical species that originate from the dissociation of a single ionic surfactant molecule. For our cHAL material, taking into account that the mean content of carboxylic moieties per molecule is 17.14, the n value may range from 1 (assuming no ionization) to 18.14 (assuming complete ionization). Substituting in eq 4 the slope value (-28.9) measured in the steeper tract of the curve in Figure 2, the Amin values at room temperature calculated from eq 5 result in 32.73 Å2 for n ) 1 and 589 Å2 for n ) 18.14. Although we cannot assess here a reliable degree of ionization at the water-air interface, the higher Amin value appears more consistent with values reported for the molecular radius of naturally occurring HAs which increases from 9.8 to 17.8 Å as the molecular weight increases from 5000 to 30000 (1). In addition to this, the area occupied by an adsorbed molecule in an interface of water-air depends principally on the size of its polar group, being for many groups (-OH, -COOH, -NH2) nearly 21 Å2. Amin of this order of magnitude may well be expected for simple molecules having one polar head. Our cHAL molecule however contains an average of ca. 51 polar carboxylic and phenol acid groups and should adsorb at the air-water interface in a more or less flat arrangement, as opposed to a vertical disposition for a simple molecule such as SDS having one polar head. Also, in the solution at pH 7 the carboxylic group should be completely ionized according to measurements performed for peat HA at the same pH (9). Both these factors should cause much larger Amin than those expected for simple molecules with one polar head. For comparison, we have performed calculations of Amin using the surface tension data published for peat HA (9). For this material, whose chemical characterization data have been included in Table 1, the reported surface tension data under the same pH and temperature conditions as for cHAL are cmc ) 2511 mg/L, γcmc ) 47.1 mN/m, and δγ/δ log Cw ) -15.03 in the steeper tract of the γ-log Cw curve. The results of our calculations, assuming complete ionization also in this case, yield Amin ) 3024 Å2. Contrary to these findings, by neglecting the use of the n Gibbs prefactor, several authors have calculated areas

in the range 45-63 Å2 for peat HA with 17000 MW (9) and 30-72 Å2 for other HAs (11-13) having molecular weights in the 1000-300000 range. These values indeed appear very small compared with the probable larger dimension of the whole polar surfactant surface exposed at the air-water interface. Table 1 shows that the above peat HA has nearly the molecular weight of our cHAL, but over double carboxylic acid content. The soil HAs (Table 1) have even higher carboxylic acid content. Looking at the carboxylic acid content in Table 1 and at the surface tension data in Table 4, it appears that the higher content of carboxylic acid groups in the other HA and HAL materials causes higher values for cmc, γcmc, and Amin relative to cHAL. The results for cHAL appear quite remarkable when compared (Table 4) with performance data for other major commercial and research surfactants. Relative to these materials, our cHAL sample seems to exhibit both low cmc and low γcmc. Only CTAB and some sedimentary HAs having molecular mass at the 300000 level (12) exhibit cmc values which are comparable with or lower than the cmc value found for cHAL. Only some modified Kraft lignin products have a lower γcmc value than that for cHAL. Aside from three commercial surfactants derived from nonrenewable source (SDBS, SDS, and CTAB), the other surfactants listed in Table 4 derive from lignin matter. For the materials recovered from pulping process wastes it should be pointed out that neither alkali nor Kraft lignin (KL) nor lignosulfonates (SL) possess a well-defined hydrophilic-hydrophobic structure, which augurs well for these products as primary surfactants. For enhanced performance, propoxylation of SL (42) and several other chemical reactions for KL (38) have been performed. Compared to these materials, cHAL has the advantage of a virtually no cost source and of not requiring any modification by synthetic chemistry. Tables 1 and 4 show that also a wide variety of HAs and HAL are available in soil, sediment, and peat and by composting. The comparison of surfactant properties between peat HA (9) and cHAL has shown how changes of the chemical nature of humic acids can affect their surfactant performance. In principle, one could isolate either from soil or from compost chemicals and/or ionic materials tailored for specific needs. In practice, compost is a more favorable source than soil both because of the higher humic acids content and because one could tune up the production process by changing the compost wastes mix and the composting time. For practical purposes, the PHE solubility data have shown how cHAL at relatively low concentration can strongly enhance the solubility of hydrophobic compounds by micelles formation in the bulk water phase. By comparison, the commercial SDBS surfactant, micellizing at 1 order of magnitude higher w/v cmc, yields the same solubility enhancement but at 1 order of magnitude higher surfactant concentration. As much of the justification for this work is the potential commercial and technological applicability of compost extracts, one final comment should be made on the feasibility of the reported cHAL extraction procedure on a larger scale. It is evident that the very high ratio of extraction fluid (50:1 v/w solution/compost ratio) does not appear attractive for large-scale operation. However, we wish to point out that in this work we have used the extraction procedure established by the Italian legislation (43) and used by other authors (44) for HAL characterization purposes. Other workers have extracted successfully HAs and HAL from soil (45) and compost (7) using a 2.5-4 v/w solution/solid matter ratio. We are therefore confident that optimization of the extraction fluid/solid ratio for large-scale operation of surfactants recovery from compost should not be a problem. In addition to this, it is not known at this point whether isolation of cHAL is really necessary in all cases. Raw compost extracts containing fulvic acids and other active organic substances

together with cHAL might show just as good or better surfactant properties for specific needs. In this case, the needed surfactant mix might be extracted with an even simpler procedure than that used in this work. We feel in conclusion that the results of this work certainly point out a new renewable source of surfactants and stimulate further intriguing work with other composts to exploit their full potential.

Acknowledgments The authors are particularly grateful to Compagnia San Paolo (Torino, Italy) and the Fondazione Cassa di Risparmio di Torino for supplying laboratory equipment. We thank Prof. E. Prenesti of the University of Torino for his advice on potentiometer titration analyses.

Supporting Information Available Experimental details for chemical and spectroscopic analyses and other pertinent data. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review August 18, 2005. Revised manuscript received November 19, 2005. Accepted December 1, 2005. ES051637R