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Formation of organo-mineral complexes as affected by particle size, ph, and dry-wet cycles.

Introduction

The formation of organo-mineral complexes protects organic matter from biodegradation (Ransom et al. 1997; Kalbitz et al. 2005) and thus is an important organic-matter stabilising process (Wagai and Mayer 2007; McCarthy et al. 2008; Ghosh et al. 2009). For a typical natural soil, organo-mineral complexes account for 50-80% of total organic carbon, 60-90% of total nitrogen, and 50-75% of total phosphate (Xiong 1983), and as such, greatly impact soil fertility. In addition, the coating of organic matter on soil particle surfaces may decrease the aggregation of mineral particles, which enhances soil porosity and consequently increases a soil' s ability to retain water (Sparks 2003).

Various mechanisms have been proposed to account for natural organic matter (NOM)--mineral particle interactions. Among these, two types have been identified as most significant: Coulomb force and van der Waals interaction (Xiong 1983; Kang and Xing 2008; Ghosh et al. 2009). While Gu et al. (1994) suggested that ligand exchange between carboxyl/hydroxyl functional groups of NOM and iron oxide surfaces was the dominant interaction mechanism, especially under acidic or slightly acidic conditions, the predominant mechanism(s) seems to be case-dependent. Yoon et al. (2004) suggested that in the inner sphere, Coulomb force was dominant (ligand exchange), while in the outer sphere, the van der Waals force was the controlling factor. Feng et al. (2005) compared NOM adsorption on kaolinite and montmorillonite at different pH levels, ionic strength, and ion valence states and found that 33% of the total interaction force could be attributed to ligand exchange, 40% to cation bridging, and 22% to van der Waals force. Although most studies have emphasised the importance of chemical bonding, some researchers indicated that physical adsorption (coating) was also important in the formation of organo-mineral complexes (Greenland 1971; Jardine et al. 1989; Ransom et al. 1997). This interaction mechanism, however, has not been extensively recognised or examined.

In the studies mentioned above, two methods were applied to study the formation of organo-mineral complexes, batch experiments and flow (miscible displacement) techniques, both of which simulate aqueous environments. For most cases in nature, however, organo-complexes are formed in terrestrial environments in which wet-dry cycling is the unique process. During wet-dry cycling, physical coating may play a significant role, but the process has not been widely studied relative to the formation of organo-mineral complexes. This study was designed to compare the formation of organo-mineral complexes in aqueous environments (sorption experiment) with that in terrestrial environments (wet-dry periodic procedure) at environmentally related pH. In addition, the fractionation of NOM resulting from organo-mineral complex formation was investigated using ultraviolet spectrometer.

Experimental

Preparation of dissolved humic acid

A dissolved humic acid (DHA) was used in this study as a type of NOM because it could be easily obtained and characterised (Sibley and Pedersen 2008). A natural soil was collected from the nonhern outskirts of Beijing, far from any industrial area. The soil sample (with organic carbon (OC) of 2.89%) was ground and sieved through a 2-mm sieve. Humic acid was extracted from the soil as described previously (Pan et al. 2007a). Briefly, a solution containing 0.1M NaOH and 0.1 M [Na.sub.4][P.sub.2][O.sub.7] was mixed with the soil. After 12 h of equilibration, the mixture was centrifuged at 1000G for 10 min and the supernatant collected. The extraction procedure was repeated three times until a light yellow extract was reached. The supernatants were filtered and collected, and the humic acid was precipitated with HCl. The precipitated humic acid was soaked in hydrofluoric acid three times, washed using distilled water until a negative test for chloride was observed using Ag[NO.sub.3], freeze-dried, and ground to <500-[micro]m panicles. An aliquot of humic acid was dissolved overnight in 2 mL NaOH (0.5 M) and then mixed with 100 mL Na[N.sub.3] solution (200 mg/L, used as biocide). The DHA was then filtered through a 0.45-[micro]m filter and kept in a refrigerator. The concentration of DHA was measured using a TOC analyzer (Shimazhu 5000A) and expressed as carbon content.

Preparation of mineral particles

The base solution-extracted soil particles were oxidised to further remove organic matter. The particles were soaked in a hydrogen peroxide ([H.sub.2][O.sub.2], 30%)/water mixture (1:3, v/v) at 40[degrees]C. The upper two-thirds of the supernatant was replaced with fresh [H.sub.2][O.sub.2] every day. After 40-day reactions, the residual particles were washed with distilled water, saturated with calcium by mixing three times with 0.5 M Ca[Cl.sub.2], and washed with distilled water until a negative test for chloride was observed using Ag[NO.sub.3]. The particles were wet-sieved and separated to panicle sizes >0.149 mm (MP1), 0.149-0.074mm (MP2), 0.074-0.050mm (MP3), 0.050-0.002mm (MP4, gravity settled and analysed), and <0.002 mm (MP5, centrifuged at 2000G). Panicles of different sizes were then freeze-dried. The residual OC contents in the treated particles are <0.08% (Pan et al. 2007b).

Adsorption of DHA on mineral particles in batch adsorption experiments

An aliquot (0.2-0.5 g) of mineral panicles (both unfractionated and fractionated) was weighed and mixed with different concentrations of DHA (2-50 mg OC/L). The solid/aqueous ratio was 1:30 (w:v). The mixture was rotated in the dark during equilibration. According to preliminary studies, two days is sufficient to reach equilibrium for all types of panicles. After equilibrium, the mixture was centrifuged at 2000G and the supernatant sampled for total OC (TOC) analysis and UV characterisation. DHA pH was adjusted to 4, 7, and 9 using 0.1 M HCl or 0.1 M NaOH. All the vials were continuously shaken during the experiment, and no precipitation of DHA was observed at any pH. The pH change before and after the equilibration was [+ or -]0.5 according to our measurement, and this change was not considered sufficient to affect the adsorption process.

Formation of organo-mineral complex during wet-dry cycles

Two grams of [H.sub.2][O.sub.2]-treated soil panicles (unfractionated) was mixed with 2 mL DHA (~500 mg C/L) stock solution, and the sludge-like mixture dried at 40[degrees]C. The dried complex was ground (<0.5mm) and 2mL DHA solution added subsequently. The wet-dry cycle was repeated until a desired amount of DHA was added. Four batches of complexes were synthesised, namely A, N, B, and M, where A, N, and B stand for organo-mineral complexes formed in acid, neutral, and base conditions, respectively, and M stands for the complexes formed after doubled wet-dry cycle times at neutral pH. To prepare M complexes, the added DHA solution was diluted two times to ~250mg C/L. Thus, to add the same amount of DHA to the mineral panicles as in A, N, and B batches, doubled wet-dry cycle times should be used. The dried samples were washed with distilled water until TOC in the water was <5 mg/L. All the complexes were ground to <0.5 mm. The DHA washed off from organo-mineral complex was collected and characterised using UV at 465 and 665 nm. The organo-mineral complexes were then measured for TOC content.

Data analysis

Three models were applied for data processing of DHA adsorption on mineral particles:

Langmuir model: S = [S.sup.0]bC/(1 + bC) (1)

Freundlich model: S = [K.sub.F][C.sup.n] (2)

Linear model: S = KC (3)

In the above equations, S (rag C/kg) and C (mg C/L) stand for DHA concentrations (OC based) in solid and aqueous phases at equilibrium; [S.sup.0] (mg C/kg) is adsorption capacity in Langmuir modelling and b (L/mg C) is the Langmuir adsorption constant; [K.sub.F] is the Freundlich adsorption constant and n is a nonlinear factor; K (L/kg) is the linear adsorption coefficient. The nonlinear regression was conducted using SigmaPlot (9.0) and statistical analysis conducted using SPSS (10.0). The performance of the three models in describing all the adsorption isotherms was compared based on [r.sup.2] adj.

Results and discussion

Adsorption isotherms of dissolved humic acid on mineral particles with different particle sizes

The adsorption of DHA on MP1 and MP2 was negligible, probably because of the low adsorption affinity of DHA on coarse sandy particles. Thus, the discussion of DHA adsorption on these two fractions will be skipped. A typical adsorption isotherm for DHA on MP is presented in Fig. 1. Because the linear model was unable to capture the adsorption isotherm properly, Table 1 presents the fitting results for the Langmuir and Freundlich models only. The standard estimation errors of the fitted parameters in Langmuir modelling are generally very high, indicating the instability of this model in describing the adsorption data. Therefore, Freundlich modelling results were adopted for further discussion. Single-point [K.sub.d] values were also calculated using the Freundlich model at different concentrations for detailed comparison.

[FIGURE 1 OMITTED]

MP3 and MP4 showed comparable adsorption to DHA, with [K.sub.d] values varying between 2.9 and 4.9 L/kg (calculated at 10 and 30mg C/L). However, adsorption coefficients were 10 times higher for the finest particles, MP5, which indicates that if the accessibilities of DHA to different particles are the same in natural soil particles, most DHA adsorption occurs on clay particles (<0.002mm). Doick et al. (2005) collected natural soils of different depths and compared TOC contents for different size fractions. Their results indicated that the particles with the smallest particle size (<0.002 ram) showed the highest TOC content. Kahle et al. (2004) indicated that soil properties such as surface area, cation exchange capacity, and the pedogenic oxide content of bulk soil mineral phases are largely determined by the clay fractions, which consequently control the adsorption of NOM on soils. Our adsorption results also indicated that the high adsorption on fine particles is an important process in stabilising organic matter.

The [E.sub.465]/[E.sub.665] ratio is considered a characteristic organic matter parameter that does not change with NOM concentration, and different [E.sub.465]/[E.sub.665] values suggest different chemical composition of various NOMs (Schnitzer and Khan 1978). Lower [E.sub.465]/[E.sub.665] levels have been shown to be related to higher aromatic content (Schnitzer and Khan 1978; Kang and Xing 2008). [E.sub.465]/[E.sub.665] values for aqueous residual DHA were significantly different from original DHA (P < 0.01), decreasing from 7.0 [+ or -] 0.2 to <6.5 after adsorption. Smaller particle size is also related to smaller [E.sub.465]/[E.sub.665] values (Fig. 2a). This change in [E.sub.465]/[E.sub.665] values is consistent with adsorption--higher adsorption is related to the extent of change in [E.sub.465]/[E.sub.665] and clearly suggests that DHA is fractionated upon sorption with the aromatic fractions left in the aqueous phase.

Because NOM is a complex of molecules with various properties, NOM molecules are fractionated after adsorption on solid particles (Balcke et al. 2002; Kang and Xing 2005; Wang and Xing 2005). Generally speaking, NOM molecules with higher molecular weight and greater humification will be preferentially adsorbed on mineral particles (Ransom et al. 1997; Namjesnik-Dejanovic et al. 2000; Specht et al. 2000; Ohno et al. 2007). Aromatic components in NOM were also reported to be adsorbed preferentially on solid particles (Murphy et al. 1990; Namjesnik-Dejanovic et al. 2000; Balcke et al. 2002; Kalbitz et al. 2005). On the other hand, some investigators observed that the aliphatic fraction was the dominant fraction in humin (mostly organo-mineral complex) (Rice 2001), and higher aliphatic content was observed for the inner layers of organic coatings on soil particles (Kang and Xing 2005), indicating a preferential adsorption of the aliphatic fraction on mineral/soil particles. Wang and Xing (2005) characterised organic matter complexed with mineral particles and provided direct evidence that the aliphatic fraction tended to be adsorbed while the aromatic fraction tended to reside in water. Using the same technology, Feng et al. (2005) observed that montmorillonite showed strong adsorption to aromatic molecules while kaolinite showed higher adsorption to aliphatic fractions. These conflicting results suggest that the interactions between organic matter and mineral particles are dependent on the properties of both NOM and solid particles. As such, the interaction mechanism should be discussed in detail for a better understanding of the formation of organo-mineral complexes.

[FIGURE 2 OMITTED]

The adsorption coefficient for NOM on inorganic particles has been shown to vary greatly from 20 to 106 L/kg (Thimsen and Keil 1998; Feng et al. 2005). The adsorption coefficients observed in our work were at the lower end of this range. Numerous factors affect DHA adsorption on mineral particles, such as dithionite-extractable iron, specific surface area, and cation exchange capacity (Kahle et al. 2004). In addition, results from our previous work (Pan et al. 2007b) indicated that 0.08% of OC resides in the treated mineral particles, which is equal to 800mg C/kg and comparable to the OC content in the complex (Fig. 1). Further, the adsorption of DHA decreased greatly because of the presence of residual organic matter.

Adsorption of DHA on unfractionated mineral particles as affected by pH

The adsorption of DHA on mineral particles was highest at pH 4, with [K.sub.d] values of 41.3 and 52.9L/kg at equilibrated aqueous phase concentrations of 30 and 10 mg/L, respectively. The fact that higher [K.sub.d] values occurred at lower concentrations compared with those at higher concentrations is a result of nonlinear adsorption. At pH 9, the adsorption coefficients decreased seven times (Table 1). At pH 7, the adsorption of DHA on unfractionated mineral particles was comparable to those for MP5, which is consistent with our previous conclusion that the adsorption of DHA on soil particles can be attributed primarily to smaller particles at high concentration of 30 mg C/L. The pH-dependent adsorption is also noted in literature (Schlautman 1992; Laor et al. 1998; Arnarson and Keil 2000).

The [E.sub.465]/[E.sub.665] ratio decreased significantly (P<0.01) after adsorption, again indicating a preferential adsorption of DHA aliphatic components on mineral particles (Fig. 2b). However, although the mineral particles showed the highest adsorption at pH 4, the decrease in [E.sub.465]/[E.sub.665] value was less distinct. This observation is different from the results obtained for the influence of mineral particle size, where higher adsorption was related to greater [E.sub.465]/[E.sub.665] change, suggesting a different adsorption mechanism may be involved under acidic conditions. The DHA solution is negatively charged under neutral (pH 7) and alkaline (pH 9) conditions as a result of deprotonation of carboxyl and hydroxyl functional groups (Pan et al. 2007a), where the molecules are stretched and opened because of inter- or intra-molecular repulsion. The selection of DHA fractions may occur during adsorption. However, at acidic pH (pH 4), the DHA carboxyl and hydroxyl groups can be protonated and the surface charge of the DHA molecules neutralised, which may facilitate self-coiling and aggregation of the DHA molecules. Although no precipitation of DHA was observed during our experiments, the aggregation of DHA molecules could decrease the availability of some components (such as aliphatic fractions) to the mineral particle surface, with the result that less DHA fractionation is observed.

Enhanced adsorption under acid conditions can also be explained by the protonation of hydroxyl groups on the mineral surface. The following chemical processes have been suggested to describe the effect of this protonation on organic matter adsorption on the particle surface (Schlautman 1992):

SOH + [H.sup.+] [??] [SO[H.sub.2]].sup.+]

[SO[H.sub.2].sup.+] + Hu - C(O)[O.sup.-] [??] SO[H.sub.2.sup.+][O.sup.-]C(O) - Hu

[SO[H.sub.2].sup.+][O.sup.-]C(O) - Hu [??] SOC(O) Hu - [H.sub.2]O

This procedure could also be viewed as a ligand exchange process. As pH decreases, the protonation of mineral hydroxyl groups is enhanced and the adsorption of organic matter increased. Kahle et al. (2004) observed competitive adsorption between NOM and phosphate, which would indicate that the reactive hydroxyl groups of clay minerals are involved in DOC sorption. Under acid conditions, the electrostatic repulsion between the mineral surface and the organic molecules decreases (Arnarson and Keil 2000), which enhances DHA adsorption.

Formation of organo-mineral complexes during wet-dry cycles

Wet samples tend to have outer-sphere adsorption of organic matter on inorganic particles, while inner-sphere coordination of organic matter and mineral surface is dominant for freeze-dried samples (Kang et al. 2008). Thus, organic matter binding onto clay minerals under dry conditions was stronger than that under wet conditions (Xiong 1983). During experiments designed to form organo-mineral complexes, more than five wet-dry cycles were ensured. Figure 3 presents the comparison between expected (calculated from the amount of OC added during the formation of organo-mineral complex) and measured OC contents ([f.sub.OC]). Positive relationships were observed for the expected and measured [f.sub.OC]s at different pH. The ratio between these [f.sub.OC]s, however, is not the same under all conditions. The ratio of complexation (100% x [f.sub.OC.sup.measured]/ [f.sub.OC.sup.expected]) was calculated and plotted in Fig. 4. The lines describe the relationships between the complexation ratio and expected[f.sub.OC]. At neutral pH, the complexation ratio decreased as more DHA was added (higher expected [f.sub.OC]), which indicates that the complexation of DHA on the mineral surface reached its 'capacity', with [f.sub.OC] hardly exceeding 0.8% (Fig. 3). In this case, complexation occurs on the mineral surface and the complexed DHA molecules do not facilitate further adsorption of DHA. For natural soils, [f.sub.OC] is generally >2% for organo-mineral complexes, indicating that other processes may be involved in the formation of natural soil particles. In natural processes, wet-dry cycles may occur millions of times. Therefore, increased wet-dry cycle times were also investigated in this study. The results indicated that as the wet-dry cycle times doubled, the complexation ratio increased 10-27% at corresponding expected [f.sub.OC] levels (the amount of added DHA). Further, the ratio increased more significantly at lower expected[f.sub.OC]s (Fig. 4), which clearly indicates the role wet-dry cycles play in the formation of organo-mineral complexes.

[FIGURE 3 OMITTED]

The lowest complexation ratio was observed under basic conditions. Only 20-40% of added DHA complexed with mineral particles, and the ratio varied little with increased DHA input. The measured [f.sub.OC] for complexes formed at pH 4 was generally higher than at pH 7 and 9 for the same expected [f.sub.OC] (Fig. 3), which is consistent with the higher adsorption of DHA on mineral particles under acid conditions in the adsorption experiment. A positive relationship was observed between the complexation ratio and expected [f.sub.OC] at pH 4 (Fig. 4). At an expected [f.sub.OC] of 1.5%, the complexation ratio reached 100%. The reason for the high complexation ratio is that the concentration of hydrogen ions increased greatly during the wet-dry cycles (because of the subsequently added acid solutions), and thus DHA may have been precipitated onto the surface of mineral particles. At the highest expected [f.sub.OC] more hydrogen ions are added to the system and a greater extent of DHA precipitation is expected. Therefore, DHA precipitation is likely the most important process in the formation of organo-mineral complexes under acid pH conditions during wet-dry cycles.

The results of UV characterisation of the organic matter washed off from the organo-mineral complex confirmed this hypothesis. As presented in Fig. 5, the organic matter washed off from organo-mineral complexes formed at neutral and base conditions showed no significant difference from original DHA, with [E.sub.465]/[E.sub.665] values in the range 6.8 [+ or -] 0.47. Increased wet-dry cycle times did not alter [E.sub.465]/[E.sub.665] either. However, organic matter washed off from acid organo-mineral complexes showed distinct [E.sub.465]/[E.sub.665] values compared with other complexes, with [E.sub.465]/[E.sub.665] values decreasing to around 2 as[f.sub.OC] increased (Fig. 5). As proposed earlier, DHA could be precipitated under acid pH conditions during wet-dry cycles whereby the lower [E.sub.465]/[E.sub.665] values result from the organic matter that was not precipitated. If so, the decreased [E.sub.465]/[E.sub.665] values found under acid conditions are not the result of adsorption, but precipitation. In addition, the organic matter that washed off was most probably from the aromatic fractions, as indicated by the low [E.sub.465]/[E.sub.665] values.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The residual organic matter after the formation of organo-mineral complexes is different for aqueous adsorption (Fig. 2b) than wet-dry cycles (Fig. 5). During the aqueous adsorption process, organic matter may be fractionated to different molecular sizes (Namjesnik-Dejanovic et al. 2000) or different functional groups (Murphy et al. 1990; Wang and Xing 2005). However, during wet-dry cycles, organic matter is typically physically attached to the mineral particles and fractionation is not significant. Organic content, however, can reach much higher levels than for aqueous organo-mineral complexes, which would indicate different organo-mineral complex formation mechanisms are involved for the different systems (aqueous and terrestrial).

Conclusions

The adsorption of DHA on soil particles was mostly contributed by the clay fraction. Aromatic content was found to be less favourable for adsorption under all pH values and particle sizes investigated in this study. Much higher[f.sub.OC]s for the complexes were observed in the wet-dry cycling methodology than the adsorption system, but DHA fractionation was not significant in wet-dry cycles. With regard to organic matter fractions, the organo-mineral particles formed in the different environments varied greatly, which may result in different environmental functions for these complexes. As indicated in a previous study, a linear adsorption of phenanthrene is observed on complexes formed during wet-dry cycles (Pan et al. 2007b). However, for complexes formed after an aqueous adsorption process, a nonlinear adsorption was observed (Feng et al. 2006). Here, we explained the differences from the viewpoint of the composition of organic matter that is complexed with mineral particles. Other environmental functions, such as the persistence of OC, porosity, fertility, and wettability, may also lead to different results. Further study is required to investigate the mechanisms controlling the formation of organo-mineral complexes (or soils) in aqueous and terrestrial environments.

DOI: 10.1071/SR10029

Acknowledgments

This research was supported by National Scientific Foundation of China (40771179, 40730737) and National Basic Research Program (2007CB407301).

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Manuscript received 21 January 2010, accepted 21 May 2010

B. Pan (A,B), S. Tao(B,D), R. W. Dawson (B), and B. S. Xing (C)

(A) Faculty of Environmental Science & Engineering, Kunming University of Science & Technology, Kunming 650093, China.

(B) College of Urban and Environmental Sciences, Peking University, Beijing 100871, China.

(C) Department of Plant, Soil and Insect Sciences, University of Massachusetts, Amherst, MA 01003, USA.

(D) Corresponding author. Email: taos@urban.pku.edu.cn
Table 1. Fitting results of DHA adsorption isotherms on mineral
particles

s.e.e., Standard error of the estimate

                         Langmuir modelling

           [S.sup.0]                           [r.sup.2]
            (mg/kg)   s.e.e.    1/b   S.e.e.      adj

MP3 (A)          146       86  17.7     19.5         0.59
MP4 (A)          233      224  45.7     63.0         0.61
MP5 (A)         1069      127   9.4      3.4         0.80
pH 4 (B)         869      398   8.4      6.3         0.77
pH 7 (B)        1685      581  52.1     26.2         0.92
pH 9 (B)         666      292  78.7     47.4         0.91

                         Freundlich modelling

              log                               [r.sup.2]
           [K.sub.F]   S.e.e.     n    s.e.e.   adj

MP3 (A)       0.86      0.26    0.80    0.24       0.63
MP4 (A)       0.23      0.41    1.23    0.35       0.56
MP5 (A)       2.29      0.07    0.42    0.05       0.87
pH 4 (B)      1.95      0.09    0.77    0.16       0.72
pH 7 (B)      1.59      0.07    0.82    0.06       0.96
pH 9 (B)      1.22      0.08    0.70    0.06       0.94

           Single-point [K.sub.d]

             C=30      C=10

                (mg C/L)

MP3 (A)       3.7      4.6
MP4 (A)       3.8      2.9
MP5 (A)      27.4     51.8
pH 4 (B)     41.3     52.9
pH 7 (B)     21.1     25.7
pH 9 (B)      6.0      8.3

(A) Adsorption experiments with different particle sizes at pH 7.

(B) Adsorption on mineral particles at different pH.
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Author:Pan, B.; Tao, S.; Dawson, R.W.; Xing, B.S.
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:1USA
Date:Dec 1, 2010
Words:5072
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