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a Agric. and Biol. Eng. Dep., Pennsylvania State Univ., University Park, PA 16802
b Soil and Water Sci. Dep., Univ. of Florida, Gainesville, FL 32611
* Corresponding author (hae1{at}psu.edu)
Received for publication September 4, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Fe-WTR >> hematite. Differences in the P-fixing capacity of the sesquioxide-dominated materials (Al-WTR, Fe-WTR, hematite) were attributed to their varying reactive Fe- and Al-hydrous oxide contents as measured by oxalate extraction. Leachate P was monitored from greenhouse columns where bahiagrass (Paspalum notatum Flugge) was grown on Immokalee soil amended with biosolids or TSP at an equivalent rate of 224 kg P ha-1 and WTRs at 2.5% (56 Mg ha-1). In the absence of WTRs, 21% of TSP and 11% of Largo cake biosolids total phosphorus (PT) leached over 4 mo. With co-applied WTRs, losses from TSP columns were reduced to 3.5% (Fe-WTR), 2.5% (Ca-WTR), and <1% (Al-WTR) of applied P. For the Largo biosolids treatments all WTRs retarded downward P flux such that leachate P was not statistically different than for control (soil only) columns. The phosphorus saturation index (PSI = [Pox]/[Alox + Feox], where Pox, Alox, and Feox are oxalate-extractable P, Al, and Fe, respectively) based on a simple oxalate extraction of the WTR and biosolids is potentially useful for determining WTR application rates for controlled reduction of P in drainage when biosolids are applied to low P-sorbing soils.
Abbreviations: Alox, Feox, and Pox, oxalate-extractable Al, Fe, and P, respectively • Al-WTR, water treatment residual generated using alum • BPR, biological phosphorus removal • Ca-WTR, water treatment residual generated in lime softening process • Fe-WTR, water treatment residual generated using iron salts • PSI, phosphorus saturation index • TSP, triple superphosphate • WTR, water treatment residual
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Unlike the case with nitrogen, leaching of P to ground water has not traditionally been a major transport process. In many soils, high P-sorbing oxide components keep leachate P levels well below eutrophication thresholds (0.01 to 0.05 mg L-1, Sims et al., 1998). While this applies in many parts of the country, vertical flux of P is potentially significant in areas with shallow ground water and coarse-textured soils with little P-sorbing capacity (Eghball et al., 1996). Areas of intense animal agriculture of the Atlantic Coastal Plain are particularly susceptible to deep leaching of P (Sims et al., 1998; Novak et al., 2000). Because leached constituents can move to surface water via lateral subsurface flow, controlling labile P in porewaters is important in managing eutrophication of surface waters.
Increased emphasis on soluble P losses from cropland has spawned the use of chemical amendments to immobilize P in soils and land-applied wastes. Aluminum, Fe, and Ca salts have been used to decrease P solubility in poultry manure and runoff from manure-amended soils (Moore and Miller, 1994; Moore et al., 1999). Because alum (aluminum sulfate) and Fe salts are used to remove turbidity and color from surface water supplies, residuals produced in drinking water treatment contain reactive hydrous oxides with substantial P-fixing capacity (Elliott et al., 1990). These water treatment residuals (WTRs) have been used to reduce P bioavailability and runoff potential in soils (Peters and Basta, 1996; Cox et al., 1997; Gallimore et al., 1999). Ippolito et al. (1999) studied the effects of co-applying WTRs and biosolids on P uptake and biomass production in range grasses. They concluded that a WTR to biosolids mass ratio of 8:1 will sorb all soluble biosolids P but higher ratios may immobilize plant-available P and induce P deficiency in crops. The relative effectiveness of WTRs in reducing labile P depends on source water characteristics, treatment operational methods (Hyde and Morris, 2000), and storage conditions prior to land application.
Several byproduct materials are generated in drinking water purification processes, but two major types of WTRs are produced in large quantities and have potential for P immobilization. Conventionally, sedimentation–flocculation processes produce residuals using either Al salts (e.g., alum) or Fe salts (e.g., ferric chloride) as the primary coagulant. These residuals are herein referred to as Al-WTRs and Fe-WTRs. The other major residual type, Ca-WTR, is produced in water-softening facilities where lime is used for hardness removal. For at least 60 yr water-softening residuals have been used for liming agricultural fields (American Water Works Association, 1951), but their use in P retention has not been widely investigated.
While the role of WTRs in reducing P in surface runoff from high-P soils has been recently researched (Gallimore et al., 1999; Haustein et al., 2000), their effect on P leaching has received limited attention. Therefore, the aim of this study was to investigate the ability of WTRs to sorb P and influence downward movement of biosolids P in a sandy soil material of low P-sorbing capacity. Batch equilibrium studies were used to investigate the influence on soluble P of WTRs incorporated at various rates into biosolids–soil suspensions. Greenhouse column experiments were conducted to quantify WTR effects on leaching of biosolids P in a crop growth environment. While P leaching is negligible for most biosolids-amended soils (Peterson et al., 1994; Elliott et al., 2002), our ultimate objective was to determine the effect of WTRs co-applied with biosolids in a system where drainage P is a justifiable concern.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Equilibration Studies
Batch equilibration studies were conducted using Immokalee sand. Selected properties are presented in Table 3
. The soil has little native P (22.4 mg P kg-1), contains only 125 mg kg-1 oxalate-extractable Fe + Al [Alox + Feox], and has a correspondingly low P-sorbing capacity. Suspensions of Immokalee soil fertilized with various P sources at equivalent PT concentrations of 100 mg P kg-1 and amended with WTRs were prepared in 0.01 M KCl at a 1:1 soil to solution ratio, using 10 g soil. The suspensions were reacted at room temperature (23 ± 2°C) for 24 h, filtered, and centrifuged, and the supernatant was filtered again (0.45 micron). The various WTR rates (0, 0.1, 0.5, 1.0, 2.0, and 5.0%, by weight) were chosen based on preliminary sorption data (O'Connor and Elliott, 2000) and effectiveness demonstrated in previous studies (Elliott et al., 1990; Lucas et al., 1994). Besides TSP, the following biosolids were used as P sources: Tarpon Springs cake, Largo pellets, and Baltimore pellets.
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All WTRs were incorporated into the Immokalee soil at a 2.5% rate, while the hematite was applied at a loading rate of 1.01%, an iron-equivalent rate to the Fe-WTR. Besides being typical of rates used in previous studies (Elliott et al., 1990; Lucas et al., 1994), a 2.5% WTR application rate of the same Fe-WTR significantly reduced P leaching with minimal negative effect on P uptake by bermudagrass (Brown and Sartain, 2000). An added advantage to this WTR loading is that the 2.5% rate of Fe-WTR (4.36 g kg-1 PT) is equivalent to an application of 220 kg P ha-1, essentially identical to the P loading rate used for the P sources (224 kg P ha-1). Thus, the leachability of the P in the WTR could be compared with that of TSP and biosolids on an equal PT basis. Leachate PT was determined by colorimetric analysis (Murphy and Riley, 1962) following persulfate digestion in an autoclave. Phosphorus analysis of undigested drainage water, from this (data not presented) and other biosolids-amended soil leaching studies (Elliott et al., 2002) showed little effect of digestion. Thus, leached P existed almost exclusively as inorganic P. Digested leachate P values are nevertheless reported to quantify total P leached, and to compare with total P applied.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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-Fe2O3) by X-ray diffraction. Loss on ignition (LOI) values for the Al-WTR and Fe-WTR were 24 and 36%, respectively. These values are too high to be reflective of the humic matter content in typical surface waters. Elliott et al. (1990) compared total organic carbon (TOC) and LOI of several WTRs and concluded that LOI overestimates organic content since water bound to hydrous oxides is also removed during ignition.
While PT contents of the Ca-WTR and hematite are negligible, those of the Al-WTR and Fe-WTR (2.79 and 4.36 g kg-1, respectively) are sufficient to affect P loadings to the columns. However, this P has limited solubility as indicted by KCl-P values of <0.41 mg kg-1 and low Mehlich 1–extractable P values. Significant portions of PT in the Al- and Fe-WTR were oxalate-extractable, suggesting that the P in these residuals exists as amorphous Al-P and Fe-P solids with limited solubility.
All biosolids had total N levels (average NT = 62 g kg-1) and P levels (average PT = 28.9 g kg-1) within the normal range of biosolids produced nationally (USEPA, 1986). Inorganic P forms dominated all P sources (approximately 80–90%, Table 2). Even the Tarpon Springs and Largo materials, generated by BPR processes, contained predominantly (78–85%) inorganic P. Major elemental concentrations (AlT, FeT, CaT, and MgT) were also representative of biosolids produced nationally, and reflected individual wastewater and sludge treatment processing. Thus, FeT or AlT concentrations were generally <10 g kg-1, unless chemicals were added to the waste stream (e.g., ferric chloride [Baltimore] or alum [Tarpon Springs] salts for P removal).
Biosolids varied widely in the amount of labile P estimated from the P extracted in the first (KCl) step of the sequential extraction (Table 2). The relatively high KCl-P of the Largo biosolids reflects BPR wastewater treatment combined with a lack of chemical addition for precipitation of metal phosphates (Elliott et al., 2002). Phosphorus extracted by KCl was shown in previous work (O'Connor and Sarkar, 2000) to reflect reasonably the relative bioavailable and leachable P for three biosolids. For example, the Largo pelletized biosolids (KCl-P = 3.5 g kg-1) provided much greater bioavailable and leachable P than the high Fe Baltimore pellets (KCl-P = 0.14 g kg-1) (O'Connor and Sarkar, 2000). Inorganic P was dominantly in Fe- and Al-associated forms (NaOH extract). The sum of sequentially extracted P forms is usually taken to represent total inorganic P. Given the dominance of biosolids PT by inorganic forms, we expected the good correlation (r2 = 0.94) found between the sum of sequential extracts and PT (Elliott et al., 2002).
The Mehlich-1 soil test was also applied to biosolids (Table 2). Previous work (O'Connor and Sarkar, 2000) suggested that Mehlich-P in biosolids was at least qualitatively related to bioavailable and leachable P, although not as indicative as KCl-P. Mehlich-P was poorly correlated (r2 = 0.13) to PT for the biosolids examined. The Pox was well correlated (r2 = 0.76) with PT, indirectly confirming the dominance of Fe- and Al-P forms in most biosolids materials (Soon and Bates, 1982; Elliott et al., 2002). Oxalate extracts noncrystalline Fe and Al and, thus, is expected to release P associated with amorphous Fe and Al solids.
Equilibration Studies
The amount of soluble P remaining after suspension equilibrium is shown in Fig. 1 through 4
for the Al-WTR, Fe-WTR, Ca-WTR and hematite, respectively. Addition of all WTRs and hematite reduced the soluble P levels, and the effect varied as follows: Al-WTR > Ca-WTR Fe-WTR > hematite. For any given amendment, differences in equilibrium soluble P values were almost entirely a function of initial P source solubility as indicated by KCl-P (Table 2). Solubility of P sources varied consistently as follows: TSP > Largo pellets > Tarpon Springs > Baltimore.
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Differences in the equilibrium soluble P supported by the three sesquioxide-based materials (Al-WTR, Fe-WTR, and hematite; Fig. 1, 2, and 4, respectively) are explicable in terms of the reactivity of the oxide components of each material. The P-sorbing potential of Al and Fe hydrous oxides is strongly influenced by their crystallinity. Freshly precipitated X-ray amorphous Fe and Al gels sorb 10 to 100 times more inorganic P than their crystalline analogues (McLaughlin et al., 1981). Noncrystalline oxides are often measured by oxalate extraction (McKeague et al., 1971). Thus, the oxalate-extractable Al + Fe [Alox + Feox] serves as a useful index of WTR P-sorbing capacity. The [Alox + Feox] for Al-WTR, Fe-WTR, and hematite were 2.7, 1.1, and 0.18 mol kg-1, respectively. The Al-WTR was therefore more effective in decreasing soluble P than Fe-WTR, while the hematite was comparatively ineffective. Clearly, total elemental analyses are unreliable predictors of P sorbing capacity: the hematite (FeT = 754 g kg-1) was considerably less effective than Fe-WTR (FeT = 329 g kg-1) for the same amendment rate. Gallimore et al. (1999) concluded that the amorphous, rather than total, Al content of WTRs determines their ability to reduce P in surface runoff.
Basta and colleagues (Basta et al., 2000; Dayton et al., 2000) reported Alox and Feox for several WTRs. The Alox content of 18 alum WTRs ranged from 0.1 to 1.8 mol kg-1, with a mean value of approximately 0.9 mol kg-1 (Dayton et al., 2000). The strong P-fixing ability of the Al-WTR in this study (Fig. 1) is consistent with its high [Alox + Feox] content (2.7 mol kg-1). The P-fixing capacity of WTRs is influenced by a host of factors. For example, the water source turbidity and coagulant dosage determine the relative proportions of surface water sediment and coagulant hydroxide precipitate in the resulting WTR. The P sorbing capacity of the WTR is expected to vary directly in proportion to its reactive hydrous oxide content. Hyde and Morris (2000) reported that the method of dewatering also influenced the ability of WTRs to decrease extractable P in amended soils.
Additions of Ca-WTR dramatically reduced soluble P concentrations from the various P sources (Fig. 3), and there was little difference in effect at rates of 0.1 to 2%. Phosphorus source solubilities were arrayed in the same order as with the sesquioxide-based materials (Fig. 1, 2, and 4), but only the TSP maintained appreciable soluble P in the presence of Ca-WTR addition. While soluble P was decreased as Ca-WTR was added, the effect was expected to be less than that observed with the other WTRs based on the paucity of Alox and Feox in the Ca-WTR (Table 1). It should be noted that the P sorption behavior of the Ca-WTR is mechanistically distinct from WTRs arising from coagulation–filtration processes where Al- and Fe-hydrous oxides are precipitated. The Ca-WTR material tended to dissolve under circumneutral pH conditions, and its alkaline nature (Table 1) resulted in higher pH values (6.5 to 7.1) for the Ca-WTR equilibrium soil suspensions, compared with the other systems (pH 4.8 to 6.4). The greater pH values and greater soluble Ca concentrations of the Ca-WTR systems probably precipitated calcium phosphates that did not form in the lower pH (Fe- and Al-WTRs) equilibration suspensions. Limited equilibrium modeling (data not presented) of the solution chemistries suggested saturation with respect to hydroxyapatite (a Ca-P solid) in the Ca-WTR systems. The important point is that removal of soluble P in the Ca-WTR system was probably a manifestation of precipitation and not solely adsorption.
Greenhouse Leaching Study
Leaching over 4 mo (cumulatively, approximately one pore volume of drainage) from columns amended with TSP and biosolids in the absence of WTRs is given in Table 4
. The data show that the P leached from biosolids is less than that from a highly soluble source like TSP. The relative amount of leachable P in biosolids was a reflection of the labile P as measured by KCl extraction. Thus, the biosolids with the most P leached (Largo cake) also had the highest KCl-P. Detailed discussion of the biosolids-P treatments in the absence of WTRs is presented elsewhere (Elliott et al., 2002).
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The PSI concept is useful for explaining why hematite had no effect on P leaching from the TSP-amended soil yet was extremely effective in suppressing P in drainage from the Largo biosolids treatments (Table 5). In the absence of WTRs, both TSP and Largo cake exhibited substantial P leaching. Because of the large Pox of the TSP (Table 2) and the modest Feox of the hematite compared with its PT (Table 1), the composite PSI for TSP–hematite is 1.6. This is well above the threshold PSI (approximately 1.1) associated with substantial P leaching (Fig. 5). The Pox of the TSP is approximately 89% of PT; however, only 36% of the PT in the Largo cake is oxalate extractable. Thus, for the Largo–hematite treatment, the composite PSI is 0.69, below the leaching threshold value (see Fig. 5). These comparisons help validate the predictive capability of the PSI of an amendment (or mixture of amendments) as a measure of potential P leaching in waste-amended soils of low P-sorbing capacity.
Although the PT content of the WTRs themselves (Table 1) is small (<4.36 g kg-1) compared with biosolids, large application rates (e.g., 56 Mg ha-1) can result in appreciable soil applications of P. Average PT content of WTRs is about 2 to 3 g kg-1 (USEPA, 1996). In this study, a 56 Mg ha-1 loading of the Fe-WTR (PT = 4.36 g kg-1) corresponded to the addition of 220 kg P ha-1. Yet less than 1% of PT in the Fe-WTR was extractable by Mehlich-1 and the P was not extracted to any significant extent by KCl (Table 1). Thus, P was expected to be nonleachable from the soil amended only with Fe-WTR. This was, in fact, the case (Table 5). Nonleachability is also predicted from the PSI of Fe-WTR alone (0.039), and clearly illustrated in Fig. 5.
The limitations of the PSI must be appreciated. First, it is most useful in systems where the characteristics of the amendment dominate P chemistry. The behavior exhibited in Fig. 5 applies to the Immokalee sand with very low P-sorbing capacity. For the predominant agricultural soils of the USA, P leaching will be negligible regardless of the amendment PSI, at least if the soil P-sorbing sites are not already saturated prior to spreading. Second, the index applies to Al- and Fe-based WTRs where sesquioxide components determine P-fixing capacity. Because of the aforementioned differences in P removal mechanisms for Ca-based WTRs, [Alox + Feox] is not a reliable measure of P-fixing capacity for softening residuals. In practice, the liming potential of Ca-based softening residuals will probably dictate application rates on acid sandy soils.
Finally, a single high WTR application rate was used in this study. The data are insufficient to ascertain if the composite PSI of WTR–biosolids combinations exhibits the same threshold behavior (Fig. 5) as the PSI of biosolids alone. This is the subject of further investigation that will be reported subsequently.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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-Fe2O3) was relatively ineffective in immobilizing P because of the lower P-sorbing capacity of crystalline versus X-ray amorphous oxides. The Ca-WTR from water softening probably reduced soluble P through precipitation of Ca-P solid phases. Its substantial calcium carbonate equivalent means its liming potential must be considered in use as a soil amendment. Because Al and Fe salts are used in wastewater treatment and solids dewatering, leaching of P is quite small for most biosolids, even when applied in excess of crop P requirements to soils with limited P-sorbing capacity. Thus, for several biosolids materials, leaching was not significantly (p = 0.05) different than controls (soil only columns) even in the absence of WTRs (Elliott et al., 2002). For biosolids and fertilizers with meager Al and Fe and appreciable leachable P (e.g., TSP and Largo cake), co-application with WTRs dramatically reduced vertical movement of P through the soil profile.
Biosolids to WTR mixing ratios based on total compositional parameters (e.g., Colorado Department Public Health and Environment, 1994) are probably poorly correlated with P bioavailability or environmental mobility. Therefore, mass-based guidelines for specific biosolids–WTR combinations (Ippolito et al., 1999) are unlikely to be generally applicable to the wide range of materials produced nationally. A quantitative approach using coagulation WTRs to reduce P flux from biosolids-amended soils should be based on ensuring sufficient reactive Al + Fe in the WTR to immobilize leachable P in the biosolids. Oxalate extraction can be used to measure both the P-fixing capacity of the sesquioxides in the WTRs and the potentially releasable P in the biosolids. The phosphorus saturation index ([Pox]/[Alox + Feox]), employed as a risk indicator of soil-P loss (Chardon et al., 2000), also appears useful for determining amendment rates of WTRs needed to control drainage P in land-based biosolids recycling programs.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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