Phosphorus removal by a synthetic iron oxide–gypsum compound

Ecological Engineering 12 (1999) 339 – 351

Phosphorus removal by a synthetic iron oxide – gypsum compound Olivier Bastin 1,a, Fre´de´ric Janssens a,*, Joseph Dufey b, Alain Peeters a a

Uni6ersite´ Catholique de Lou6ain, Laboratoire d’Ecologie des Prairies, Place Croix du Sud 5, Bte 1, 1348 Lou6ain-la-Neu6e, Belgium b Uni6ersite´ Catholique de Lou6ain, Unite´ des Sciences du Sol, Place Croix du Sud 2, Bte 10, 1348 Lou6ain-la-Neu6e, Belgium Received 30 January 1998; received in revised form 11 August 1998; accepted 2 September 1998

Abstract Phosphorus pollution is a major concern for soil and water management. This study assesses the phosphate and organic phosphorus removal capacity of an iron oxide – gypsum compound (named OX) in batch trials. Phosphate solutions ranging from 0.001 to 10 mg P l − 1 were tested and OX proved to be an effective fixing agent. Solutions with different ionic strengths did not affect this reactivity. Phosphate removal was not altered by pH values between 4 and 8, but increased significantly with higher values. Near-completion of this reaction was observed after some minutes. The combined effects of precipitation with calcium (gypsum) and sorption onto the oxide explain these interesting properties. The phosphate removal capacity was demonstrated on field samples. The compound also promoted the hydrolysis of a model organic phosphorus molecule. Contact of OX with solutions with pH values between 4 and 10 did not alter its stability but caused pH levelling to neutral values. Other secondary effects involve sulfate and calcium release. OX reactivity with different phosphorus species under various conditions is interesting for application to water and soil remediation processes aiming to control phosphorus pollution. © 1999 Elsevier Science B.V. All rights reserved.

* Corresponding author. Tel.: +32 10 473006; fax: + 32 10 472428; e-mail: [email protected] 1 Present address: SmithKline Beecham, Wastewater Treatment Plant, Shewalton Road, Irvine KA11 5AP, UK. 0925-8574/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0925-8574(98)00077-9

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Keywords: Phosphorus; Iron oxide; Gypsum; Sorption; Precipitation; Wastewater treatment; Soil management

1. Introduction Iron and aluminium oxides are known for their phosphorus-retention capacity in soils and sediments (Holtan et al., 1988; Schwertmann, 1990). Their reaction with phosphate was described from a fundamental point of view in several papers. Baldwin et al. (1996) also showed that iron oxides hydrolyse organic phosphorus species and Yee (1966) found that aluminium oxides remove polyphosphates from solution. Applied research concerning iron and aluminium oxides has been carried out for some decades, often focusing on environment conservation and pollution control. Among these, freshwater eutrophication control focuses mainly on phosphorus, an element considered limiting for growth. Relevant actions against this type of pollution are classified into two categories (Meher and Benndorf, 1995): either the eutrophic water is directly treated or phosphorus sources are addressed. Regarding direct treatments, many compounds have already been added to lakes and reservoirs: fly ash, lime, gypsum (Higgins et al., 1976), slag (Yamada et al., 1986) and different salts of iron and aluminium (Meher and Benndorf, 1995). These treatments aim principally to remove phosphorus from water by direct reaction and/or to prevent its release by sediments after settling. Wastewater is one of the most obvious external sources of phosphorus. Different biological or chemical processes allow one to cope with discharge consents (1–2 mg P l − 1) implemented in the European Union (Cooper et al., 1994). Nevertheless, in some regions, more stringent measures (from 0.8 to 0.5 mg P l − 1) are applied to control lake and reservoir eutrophication (Galarneau and Gehr, 1997; Matsche´, 1997). For the purpose of polishing existing treatments, bench-scale studies were carried out on aluminium oxides with batch (Huang, 1977) and column (Neufeld and Thodos, 1969) experiments. Pilot-scale studies took place (Yee, 1966) and activated alumina filters were commissioned for removing phosphorus from small tributaries (Bernhardt, 1983). In Quebec, sludge containing aluminium hydroxides was studied as a polishing agent in wastewater treatment plants (Comeau et al., 1997; Galarneau and Gehr, 1997). Constructed wetlands usually become phosphorus-saturated within a few years and start releasing excessive quantities of phosphate (Richardson, 1985). Different substrates were tested in pilot studies, such as crushed limestone (Hardman et al., 1993) and shale (Drizo et al., 1997). The phosphorus retention capacity of constructed wetlands occurs partly as a consequence of reactions with iron compounds (Brix, 1995). According to Jenssen et al. (1991), addition of various iron and aluminium compounds would enhance phosphorus removal in these processes. A non-negligible share of phosphorus input in waters comes from agricultural non-point sources, such as losses in surface runoff and drainage (Sharpley and

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Withers, 1994). Different compounds were tested in order to reduce the excess of bioavailable phosphorus in soils: drinking water treatment sludge containing iron or aluminium (Elliot and Dempsey, 1991), cement kiln dust, treated bauxite red mud (Peters and Basta, 1996), calcium carbonate, gypsum, ferrous sulfate and alum (Anderson et al., 1995). Another, less water-oriented, application of a phosphorusfixing compound concerns the biodiversity of herbaceous associations, which is increased by a reduction of phosphorus availability in soils (Janssens et al., 1997). Prior to this study, an iron oxide–gypsum compound (named OX) was selected for its potential phosphorus removal abilities and a maximum valorisation of the raw materials utilized for its synthesis. Potential applications were as described and range from lake remediation, wastewater treatment polishing and wetland upgrading to soil management. Consequently, the range of environmental conditions involved may vary considerably, both as a function of the application and time. Hence, this paper describes the spectrum of conditions allowing effective phosphorus removal by OX. Investigations focused principally on phosphate, which is directly bioavailable. Nevertheless, other phosphorus fractions cannot be neglected as they can be hydrolyzed through abiotic and/or biotic reactions (Holtan et al., 1988).

2. Methods

2.1. Oxides synthesis and reaction with phosphate The OX compound was synthesised by dissolving 50 g of Fe2(SO4)3 · nH2O and 25 g of Ca(OH)2 in 50 ml distilled water while stirring for 5 min. The suspension was then dried at room temperature and the powder was thoroughly crushed. Both chemicals were chosen because of their utilisation in wastewater treatment (Metcalf&Eddy, 1991). The material obtained was scanned through X-ray diffraction (XRD) and showed gypsum peaks (CaSO4 · 2H2O). The iron oxide phase was a poorly ordered structure, probably ferrihydrite with Fe5HO8 · 4H2O as its formula. OX composition was 14.2% Fe and 15.4% Ca (dry weight). Two other iron oxides were used as references. The first (H) was obtained from a steel factory (COCKERILL, Chaˆtelineau, Belgium) and was identified through XRD as hematite (aFe2O3). The second oxide (Fh) was synthesised by stirring stoichiometric quantities of FeCl3 and Ca(OH)2 in water. XRD analysis did not show crystalline peaks and Fh was assumed to be a ferrihydrite. The reactivity of the three oxides was estimated with a solution of 200 mg P l − 1 as KH2PO4 and 0.1 M KNO3 (inert electrolyte). Reactions were carried out by stirring 50 ml of solution with 2 g oxide l − 1 for 24 h in 100 ml polyethylene flasks at 20°C. The resulting phosphate concentration was measured by a colorimetric method (ammonium molybdate, l= 660 nm) after filtration on membrane filters (0.2 mm). There were three replicates.

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2.2. OX reaction with synthetic phosphate solutions Phosphate solutions were prepared by dissolving KH2PO4 and KNO3 in distilled water. When not specified, reactions were carried out by stirring 50 ml of solution with OX for 24 h in 100 ml polyethylene flasks at 20°C. Radioactive labelling of solutions with 32P (PBS11 from Amersham) before OX addition allowed an accurate measurement of P removal. After reaction, the solid phase was separated from the solution by filtering 1 ml on syringe membrane filters (0.2 mm). Seven millilitres of scintillation liquid were added to the filtrate and the radioactive activity was measured by scintillation counter. Oxide reactivity was obtained by calculating the difference between the phosphate concentration of solutions with or without oxide. These values were obtained by comparing resulting radioactive activities. All the experiments involved three replicates. The experiment addressing phosphate concentration involved four solutions of 0.01 M KNO3 (0.01, 0.1, 1 and 10 mg P l − 1) and three OX loadings (0.002, 0.02 and 0.2 g l − 1). Solutions regarding pH influence contained 10 mg P l − 1 and 0.01 M KNO3. The pH was adjusted to a value ranging from 4 to 10 by adding diluted HNO3 or KOH, the reaction involved 0.25 g OX l − 1. The influence of time was assessed by reacting 80 ml of a phosphate solution (10 mg P l − 1 and 0.01 M KNO3) with 0.15 g OX l − 1 for up to 8 days. Samples of suspension were taken and filtered at regular time intervals. The ionic strength effect was studied by means of four solutions of 10 mg P l − 1 (0.001, 0.005, 0.01 and 0.1 M KNO3) reacting with 0.25 g OX l − 1.

2.3. OX reaction with field samples Two samples of water were collected in different parts of the Lake of Bu¨tgenbach (Belgium), chosen for its strong eutrophication symptoms. Three influent streams were also selected as sampling points: a brooklet receiving raw wastewater from the village Berg, the effluent of a small biological wastewater treatment plant (Worriken), and the Warche River. Conductivity, phosphate [Pi] and total phosphorus [Ptot] concentrations were estimated within a few hours with a colorimetric method (ascorbic acid, l = 890 nm), after filtration on 0.2 mm membrane filters for phosphate, and after acid hydrolysis for Ptot. The pH value was measured in the field. All these results are displayed in Table 1. Reactions were carried out by stirring 50 ml of water with OX for 24 h in 100 ml polyethylene flasks at 20°C. The amounts added to the samples ranged from 0.002 to 4 g OX l − 1, depending on the initial phosphate content. The resulting phosphate concentration was indirectly calculated by adding 32P to the samples 12 h before OX addition (see earlier). There were three replicates.

2.4. Reaction with organic phosphorus This method has been described by Baldwin et al. (1996). It assesses the ability of iron oxides to mediate the hydrolysis of a model organic phosphorus compound.

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This short experiment involved 50 ml of a solution 0.18 mM of p-nitrophenyl phosphate and Tris – HCl buffer (pH 8.8) which reacted with 1.2 g OX l − 1 to yield p-nitrophenol. The reaction took place in 100 ml polyethylene flasks stirred for 24 h at 20°C. Suspensions were then filtered on membrane filters (0.2 mm). There were three replicates. Control experiments showed negligible adsorption of nitrophenol to OX under the conditions used.

2.5. Secondary effects and stability Nitric acid and potassium hydroxide were added to distilled water to prepare four solutions with pH values ranging from 4 to 10. Reactions were carried out by stirring 50 ml of these solutions with 4 g OX l − 1 for 24 h in 100 ml polyethylene flasks at 20°C. The suspensions were then filtered on membrane filters (0.2 mm) and the conductivity of the filtrate was measured. The concentrations of Ca, Fe and three heavy metals (Mn, Cu, and Cr) were also estimated by plasma emission spectroscopy. The effect of OX on pH was evaluated by monitoring this factor during the experiment assessing the influence of pH on reactivity (see earlier). There were three replicates.

3. Results and discussion

3.1. Comparati6e reacti6ity of the three iron oxides In Fig. 1, H (industrial oxide), which is very crystalline, displays a lower reactivity than the two poorly ordered phases, Fh (synthesised from FeCl3) and OX. The reaction between iron oxides and phosphate involves an initial adsorption by ligand exchange with surface OH groups. This rapid step is followed by a slow process which is said to involve diffusion through pores or defect sites (Parfitt, 1989). The overall reaction is strongly influenced by the oxide structure. Because of the mechanisms involved, poorly ordered iron oxides are more reactive with phosphate than their crystalline counterparts (McLaughlin et al., 1981; Borggaard, 1983; Parfitt, 1989). The three compounds tested in this experiment are not Table 1 Characteristics of field samples Sample

pH

Conductivity (mS cm−1)

[Ptot] (mg P l−1)

[Pi] (mg P l−1)

Lake 1 Lake 2 Warche Berg Worriken

6.99 7.04 6.05 7.08 5.43

1490 700 1190 120 270

0.10 0.07 0.43 0.58 4.50

B0.02 B0.02 0.03 0.37 2.77

[Ptot], total phosphorus concentration; [Pi], initial phosphate concentration.

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Fig. 1. Comparative reactivity of three iron oxides after 24 h reacting with a phosphate solution (200 mg P l − 1). H, industrial iron oxide; OX, iron oxide – gypsum compound; Fh, iron oxide synthesized from feric chloride.

exceptions. On the other hand, calcium ions resulting from gypsum dissolution may induce phosphate precipitation and affect OX removal efficiency. In this case, the high reactivity is coupled with a maximum valorisation of raw materials. Additionally, washing the oxide for removal of non-reactive and undesired precipitates is not required.

3.2. Factors affecting OX reaction with phosphate Table 2 displays the resulting phosphate concentration [P] as a function of OX loading [OX]. Fig. 2 shows phosphate removal (R) plotted against phosphate resulting concentration. Two points should be highlighted. First, it can be stated that, within the conditions of this experiment, OX can remove phosphate from solutions of 0.001 – 10 mg P l − 1. These solutions cover the range of concentrations encountered in natural or sewage water and in soil solutions. Secondly, phosphate removal, and consequently the OX loading required, depends on phosphate concentration. Eq. (1) is obtained from Fig. 2 and describes the empirical relationship between phosphate removal and the resulting phosphate concentration. R =11.1[P]0.17

(1)

Table 2 Phosphate concentration in synthetic solutions after 24 h of treatment with different OX loadings [OX] (g l−1)

[P] (mg P l−1)

0.000 0.002 0.020 0.200

10.000 10.000 9.688 6.454

1.000 1.000 0.811 0.009

0.100 0.086 0.004 N.D.

0.010 0.002 N.D. N.D.

[OX], OX added to the solution; [P], resulting phosphate concentration; N.D., below the detection limits of the method.

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Fig. 2. OX reactivity as a function of resulting phosphate concentration after a 24 h reaction.

where R is the phosphate removal (mg P g − 1 OX) and [P] is the resulting phosphate concentration (mg P l − 1). Consequently, within the conditions of the experiment, the OX loading required to decrease the phosphate concentration from a given initial value to a desired concentration can be calculated by Eq. (2). [OX]= ([Pi]− [P])/(11.1[P]0.17)

(2)

where [OX] is the OX loading (g OX l − 1), [Pi] the initial phosphate concentration (mg P l − 1) and [P] the resulting phosphate concentration (mg P l − 1). These equations cannot be used as mechanistic models, but are useful in comparing results and planning future applications. For example, Parfitt (1989) carried out a similar experiment with a natural ferrihydrite where [P]=4.7 mg P l − 1 corresponded to a removal, R, of 5.9 mg P g − 1 oxide. As a comparison, the value calculated with Eq. (1) is R= 14.4 mg P g − 1 OX for the same [P]. As previously stated, in addition to the oxide structure, the reactivity is influenced by the presence of calcium sulfate (gypsum), soluble to some extent in water. Sulfate can be adsorbed on iron oxides in a similar way to phosphate, but less strongly (Parfitt and Russell, 1977). Kuo (1986) showed that there is no obvious effect of metal sorption on the phosphate reaction with hydrous ferric oxides. Consequently, calcium and sulfate ions are not likely to interact directly with phosphate sorption; their effect on ionic strength will be discussed later. However, Ca2 + ions may react in other ways. The precipitation of calcium phosphate yields different compounds such as hydroxyapatite (Ca5[PO4]3OH). These reactions are commonly influenced by the nature of the seed crystal, pH value, phosphate and calcium ion concentrations (Momberg and Oellermann, 1992). Moreover, an interaction with the natural bicarbonate alkalinity can take place to precipitate CaCO3. The mechanisms of phosphate reaction with calcium carbonate vary from a sorption (low P concentrations) to a precipitation of calcium phosphate (higher concentrations) at the surface (Holtan et al., 1988). The compounds resulting from the phosphate reaction with OX were not analyzed, but it can be reasonably

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assumed that OX reactivity relies on sorption (oxide) and precipitation (calcium). The relative significance of these two processes depends on the phosphate concentration, among other factors. Further experiments should investigate the reversibility of phosphate fixation for re-use prospects. The influence of pH on P removal (R) is clearly demonstrated in Fig. 3. The reactivity, which was almost constant between pH 4 and 8, increased significantly at higher values. Adsorption of phosphate onto iron oxides usually decreases more or less sharply with an increase in pH (Hingston et al., 1972; Ryden et al., 1977a). The opposite results in this experiment can be interpreted by means of the second component, calcium sulfate. According to its solubility product, a relatively high pH value is required to precipitate significant amounts of calcium phosphate. Lake remediation experiments (Higgins et al., 1976) showed that only higher pH allows effective soluble phosphorus removal by gypsum. In the case of OX, a higher pH may improve the yield of precipitation reactions involving calcium and phosphate and then compensate the decrease in oxide reactivity. Under acidic conditions, the formation of calcium phosphate is greatly disadvantaged. The oxide is then the major phosphate fixating agent, with an increasing efficiency. The effect of pH on the stability of OX components will be discussed later in this paper. The hybrid composition of OX allows an effective phosphate removal on a large range of pH values. Consequently, even more categories of water and soil may qualify for OX treatment, especially when subject to relatively rapid changes in pH. As an example, this may prove extremely significant in eutrophicated lakes, where algal blooms cause dramatic and transient rises in pH (Meher and Benndorf, 1995). The relationship between reaction time and resulting phosphorus removal (R) is not displayed in this paper, as the removal took place immediately after OX addition. Effectively, R = 14 mg P g − 1 OX was obtained after the first measurement (15 min) and this result did not change significantly during the next 8 days. According to Ryden et al. (1977b), a two-phase reaction takes place between iron oxides and phosphate. First, the adsorption onto the surface yields a very rapid decrease in phosphate, as in this trial. Second, a slow increase in phosphorus

Fig. 3. Influence of pH on phosphate removal by OX after 24 h reacting with a solution 10 mg P l − 1.

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Table 3 Effect of electrolyte concentration on phosphate removal by OX (24 h reaction) KNO3 concentration (M)

Ionic strengtha (M)

Reactivity (mg P g−1 OX)

0.100 0.010 0.005 0.001

0.104 0.014 0.009 0.005

14.5 15.0 14.6 14.7

a

Including KNO3 and assuming complete calcium sulfate dissolution.

fixation arises from a shift to a sorption–diffusion step. This phenomenon is probably not significant enough to be detected in this case. As far as the formation of calcium precipitates is concerned, initial precipitations may be followed by either dissolution – reprecipitation or rearrangement reactions. The influence of these processes on reaction kinetics is not easily identified. The relatively high removal obtained after a short reaction time is a very interesting point for applications in water and soils. As shown in Table 3, an increase in inert electrolyte concentration (from 0.001 to 0.1 M KNO3) did not significantly alter OX reactivity. As will be calculated in later, without phosphate as a precipitating agent, most of the calcium contained in OX dissolves in water. Hence, the maximum calcium concentration arising from OX can be easily estimated. The result (Table 3) shows that, in this experiment, the OX contribution to ionic strength is not significant in comparison with variations induced by KNO3. Consequently, the results of this experiment should be explained by reaction mechanisms and not a ‘levelling’ of ionic strength by OX. Usually, the reactivity of iron oxides is said to increase with the ionic strength because of its influence on surface charges (Ryden et al., 1977a). On the other hand, precipitation reactions are not advantaged by an increase in ionic strength. Opposite variations in reactivity may then compensate each other. Under these experimental conditions, we can state that the ionic strength does not influence the reactivity of the compound. Once again, this result is interesting for potential OX applications.

3.3. OX reaction with field water samples When considering the samples, named Lake 1, Lake 2 and Berg, resulting phosphate concentrations always equalled zero, even without adding OX. Variations in water properties after sampling (pH, redox potential, carbonates equilibrium, etc.), leading to new phosphate-fixating phases such as colloids, may explain this finding. Further trials involving filters with greater pore diameter confirmed this hypothesis (unpublished data). This emphasises the need for cautious analysis when dealing with this method and field water samples. Short-term tests without using 32P may be more appropriate in this case. Results for the Warche river and the pre-treated wastewater (Worriken) are displayed in Table 4. They show that OX is effective in fixing phosphate from these

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Table 4 Phosphate removal from field samples after 24 h reacting with OX Sample

[Pi] (mg P l−1)

[P] (mg P l−1)

[OX] (g l−1)

R (mg P g−1 OX)

Rcalc (mg P g−1 OX)

Warche

0.034 0.034 2.771 2.771

0.001 N.D. 0.849 0.200

0.02 0.20 0.10 0.20

1.7 0.2 19.2 12.9

3.4 – 10.8 8.4

Worriken

[Pi], initial phosphate concentration; [P], resulting phosphate concentration; [OX], OX added to the solution; R, phosphate removal calculated by means of experimental results; Rcalc, phosphate removal calculated by means of Eq. (1); N.D., below the detection limits of the method.

samples. Phosphate removal R (mg P g − 1 OX) can also be calculated by means of Eq. (1) (Table 4). In the case of Warche, OX was less effective than expected by calculation. The resulting phosphate concentrations are close to zero. This means there was an OX excess compared to the available phosphate and explains the difference. Concerning Worriken samples, monitored phosphate removal was higher than expected by calculation. None of the factors influencing the fixation and quoted in the previous section help to interpret this result. Either these can be classified as not significant (ionic strength) or the conditions are almost the same (initial pH, time). On the contrary, a lower removal was expected because of likely interferences by other chemical species present in these samples. Effectively, the specific adsorption of some inorganic anions (e.g. silicate) and organic compounds (e.g. humic acids) on iron oxides can lead to the desorption of phosphate (Sibanda and Young, 1986; Borggaard et al., 1990). Moreover, Anderson et al. (1995) noticed that calcium phosphate precipitation could be inhibited by organic acids.

Fig. 4. Model organic phosphate in solution after 24 h reacting with OX.

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3.4. OX reaction with organic phosphorus In this experiment, p-nitrophenol detected in solution gives an estimation of the organic phosphorus hydrolysed by OX. Fig. 4 shows the organic phosphorus content of a solution before and after OX treatment. Under these conditions, OX promoted hydrolysis of p-nitrophenyl phosphate, a model organic phosphorus. These results are in accordance with those of Baldwin et al. (1996) studying iron-oxide impregnated filter papers. The influence of calcium sulfate is difficult to evaluate at this level. This trial showed that treatment of soils or waters by OX would not only address phosphate but also other phosphorus species, leading to a better total phosphorus removal. Moreover, Yee (1966) demonstrated that aluminium oxides, structurally close to iron oxides, removed different polyphosphates from water. The potential for OX reactivity with other phosphorus species should be further determined.

3.5. Secondary effects and stability In this experiment, if the oxide part of OX were to dissolve totally, the maximum iron concentration in solution would be 568 mg Fe l − 1 (OX concentration ×Fe content). The values monitored were below 0.2 mg Fe l − 1, the detection limit of the technique utilized. Hence, oxide dissolution in solutions at pH values ranging from 4 to 10 is considered as negligible. This is important in optimizing OX reactivity and preventing iron release in the environment. The same calculation can be carried out for calcium, the maximum concentration being 616 mg Ca l − 1. In this experiment, calcium concentrations of solutions containing 4 g OX l − 1 were close to this maximum value (Ca2 + =570 mg l − 1) and did not vary significantly with pH from 4 to 10. This means that most of calcium sulfate contained in OX dissolves in water, improving OX reactivity but also releasing ions. This is further confirmed by the constant value obtained for conductivity: 1.5 mS in OX solutions at different pH. Calcium sulfate is a neutral salt and should not alter the pH. Nevertheless, all the resulting pH values were levelled to values close to 7.5, this result probably arising from residual raw materials. Concentrations of the three heavy metals tested in this experiment (Cu, Cr, and Mn) were below the detection limit (0.02 mg l − 1). In applying OX, the impact of pH levelling, calcium and sulfate release on the environment should be taken into account, but is not likely to be a major issue.

4. Conclusion This paper describes phosphorus removal by OX under different experimental conditions. The combination of iron oxide and gypsum leads to very interesting properties for water and soil applications. Effectively, environmental conditions influence the yield of phosphate reaction with both components in different, and even opposite, ways. This allows a large spectrum of applications, as sorption and precipitation compensate each other. Moreover, these properties allow a maximum

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valorisation of the raw materials utilized for OX synthesis The reaction of OX with other phosphorus species also improves total phosphorus removal. Further experiments should involve pilot studies for water and soil treatments. These should determine the processes efficiencies and the costs involved.

Acknowledgements Most of this study was funded by Electrabel. The authors wish to thank particularly J.-P. Germeau and B. Platiau (Electrabel), C. Allard and A. Iserentant for technical assistance in some experiments.

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