WO2015169682A1

WO2015169682A1 - Method for removing phosphate from water fractions

Info

Publication number: WO2015169682A1
Application number: PCT/EP2015/059487
Authority: WO
Inventors: Arie Cornelis Besemer; Vincenzo Roberto CALDERONE; Richard Johannes VAN DUIN
Original assignee: Biaqua B.V.
Priority date: 2014-05-08
Filing date: 2015-04-30
Publication date: 2015-11-12

Abstract

The invention pertains to a method for removing phosphate from a phosphate-containing water fraction comprising the steps of contacting a phosphate-containing water fraction with an adsorbent, the adsorbent comprising a complex of Fe (III) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequivalent carboxylate per gram, and withdrawing an effluent water fraction from the adsorbent, wherein the complex of Fe (III } and oxidised cellulose has an Fe(III) content of at most 70 wt. %, expressed as metallic iron, per gram of oxidised cellulose. It has been found that the use of this specific adsorbent makes it possible to obtain effluent water fractions with very low phosphate contents. Further, it has been found that the adsorbent has a high adsorption capacity, expressed in mg of phosphate per gram of adsorbent. The adsorbent can be regenerated in an efficient manner to allow re-use. It is based on a biobased, degradable material.

Description

Method for removing phosphate from water fractions

The present invention pertains to a method for removing phosphate from water fractions .

Phosphate is present in many water fractions, including waste water and water derived from water cleaning operations . As phosphate is an important nutrient for microorganisms, its presence may contribute to the growth of microorganisms.

Therefore, phosphate removal is required to prevent bacterial growth .

A particular problem with bacterial growth is that even minor growth of microorganisms, a process which is sometimes also indicated as biofouling, may interfere with further

processing of the water streams. For example , water streams are often subjected to treatment in membrane operations, e.g., membrane filtration or reverse osmosis. The occurrence of even minor amounts of biofouling in apparatus provided with a membrane will severely affect operation thereof.

Therefore, there is need in the art for methods for removing phosphate from aqueous water streams to prevent or reduce the occurrence of biofouling, in particular in subsequent

membrane operations .

Various methods to effect phosphate removal are known in the art .

WO90/05705 describes a method for the removal of phosphate from water wherein water is treated with an iron (II)

compound, e.g., in the form of iro (II) sulphate or iro (II) chloride in the presence of solid particles, which results in the precipitation of iron phosphate onto the particles. The iron phosphate containing particles can then be removed through filtration. EP1764348 describes the use of thermophilic ferritin in the removal of phosphate from water fractions .

US2008 / 007 6956 describes a method for removal of anions such as phosphate from water fractions using a lignocellulose- based anion adsorbing material, which is obtained by the steps of pelletising a lignocellulose, and contacting it with an acidic solution of an iron or aluminium salt, followed by an alkaline treatment to fixate the compounds onto the lignocellulose. The inventors have also published an article on this subject (Kim et al . , Journal of Environmental Science and Health Part A, 41:87-100, 2006).

Eberhardt et al . (Bioresource Technology 97 (2006) 2371-2376) describe the removal of phosphate from stormwater runoffs by contacting it with refined aspen wood treated with 4 wt . % carboxymethyl cellulose and 12 wt . % ferrous chloride.

There are various problems with the methods described in the art. More specifically, it has been found that there is need in the art for a method for removing phosphate from water fractions which allows phosphate removal to a very low level and which makes use of an adsorbent with attractive

properties. The present invention provides such a method.

The invention pertains to a method for removing phosphate from a phosphate-containing water fraction comprising the steps of contacting a phosphate-containing water fraction with an adsorbent, the adsorbent comprising a complex of Fe (III) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequivalent (peq) carboxylate per gram, and withdrawing an effluent water fraction from the adsorbent, wherein the complex of Fe(III) and oxidised cellulose has an Fe (III) content of at most 70 wt.%, expressed as metallic iron, per gram of oxidised cellulose . It is noted that the method of the present invention differs from the method described in Eberhardt et al at least in that in Eberhardt Fe (II) chloride is used, rather than Fe(III) as is used in the present invention. Further, in Eberhardt the amount of CMC is the adsorbent is relatively low. This leads to a ratio between Fe and carboxymethylcellulose which is much higher than that used in the present invention.

It has been found that the use of the specific adsorbent according to the invention has a number of advantages. In the first place, it was found that it makes it possible to obtain effluent water fractions with very low phosphate contents, as will be discussed in more detail below. Further, it has been found that the adsorbent has a high adsorption capacity, expressed in mg of phosphate per gram of adsorbent. This is important as it determines the size of the adsorption unit. Further, it has been found that the adsorbent used in the present invention can be regenerated in an efficient manner to allow re-use. Further, the adsorbent is based on a

biobased, degradable material which is attractive from an environmental point of view. Further advantages of the present invention will become clear from the further

specification . The invention will be discussed in more detail below.

The water fraction to be treated with the method according to the invention is a phosphate-containing water fraction. In general, the water fraction has a phosphate content of at least 20 ppb, in particular at least 50 ppb, more in

particular at least 100 ppb. The phosphate content will generally be below 1000 ppb, more specifically below 500 ppb. Within the context of the present specification, the term phosphate encompasses organic and inorganic phosphates, including orthophosphate and polyphosphate. The phosphate content can be determined using the phosphate-molybdenum method, which is well known in the art.

Other parameters of the water fraction to be treated with the process according to the invention are generally not

critical .

The water fraction to be treated will generally have a pH around 7, e.g. in the range of 6 to 7.5.

The water fraction to be treated may have a variable salt content. Its conductivity is generally in the range of 20-100 mS/m, in particular in the range of 40-70 mS/m.

The water fraction to be treated may, e.g., have a nitrate content in the range of 0.1 to 50 mg N/1, in particular 1-20 mg N/1.

The water fraction to be treated may originate from various sources. In one embodiment it is derived from a waste water treatment plant. The water fraction can be subjected to conventional pretreatment steps to remove contaminants.

Examples of suitable pretreatment steps include filtration and ultrafiltration.

The adsorbent used in the present invention comprises a complex of Fe (III ) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequivalent carboxylate per gram.

It has been found that the use of oxidised cellulose with a carboxylate content of at least 200 peq/g (microequivalent carboxylate per gram) is a key feature of the present

invention. It is preferred for the carboxylate content to be higher, e.g. at least 300 peq/g, in particular at least 400 eq/g, as a higher carboxylate content makes for increased phosphate adsorption. As a maximum for the carboxylate content, a value of at most 2000 peq/g may be mentioned, more in particular a value of at most 1000 peq/g. The carboxylate content may be determined by methods known in the art, e.g., by conductometrie titration or FTIR (Fourier Transform Infrared spectroscopy) . Oxidised cellulose is known in the art, and commercially available. It can be obtained by subjecting a cellulose, or a cellulose-based material to an oxidation step. This can be effected using various types of oxidising agents in various ways, e.g., using oxidising agents like hydrogen peroxide or other peroxide compounds, nitrogen tetroxide, whether gaseous or in solution such as phosphoric acid solution, periodate, leading to the corresponding dialdehyde derivative, followed by oxidation to the dicarboxy cellulose with sodium chlorite, optionally in the presence of hydrogen peroxide,

hypochlorite, hypobromite, or hypoiodite compounds, organic oxidising agents like TEMPO ((2,2,6, 6~tetramethylpiperidin-l- yi) oxidanyl) , or 4-hydroxy-TEMPO, and combinations thereof. Suitable combinations are reaction with hypochlorite, hypobromite, or hypoiodite, or other oxidising compound in the presence of TEMPO. The use of 4-hydroxy-TEMPO, in

particular at a pH of 3-4 as such without any further

oxidising agent is also possible. Methods for manufacturing oxidised cellulose require no further elucidation here. The complex of Fe (III) and oxidised cellulose generally has an Fe (III) content of at least 1 wt.%, expressed as metallic iron, per gram of oxidised cellulose. If the Fe (III) content is too low, the adsorption capacity of the adsorbent will be insufficient. It may be preferred for the Fe (III) content to be at least 5 wt.%, expressed as metallic iron per gram of oxidised cellulose, more in particular at least 10 wt.% expressed as metallic iron per gram of oxidised cellulose. The Fe (III) content will be at most 70 wt.%, expressed as metallic iron per gram of oxidised cellulose. When more iron is present, the accessibility of the additional iron will be limited, and it will therefore only have a limited

contribution to the phosphate removal. It has been found that an Fe (III) content of at most 70 wt.%, expressed as metallic iron per gram of oxidised cellulose, is sufficient to obtain the desired phosphate adsorption, and that the addition of further Fe (III) is not required. It may be preferred to use even lower Fe (III) contents, e.g., at most 50 wt.%, more in particular at most 30 wt.%, in some embodiments at most 20 wt.%.

In one embodiment of the invention, where the particle size is at least 100 microns, as discussed in more detail below, it may be particularly preferred for the Fe (III) content to be in the range of 1-30 wt.%, in particular 5-20 wt .

The complex of Fe (III) and oxidised cellulose can be obtained by conventional methods. For example, oxidised cellulose may be contacted with an aqueous solution of an Fe (III) salt, e.g., a sulphate, nitrate, or chloride salt, resulting in adsorption of Fe (III) onto the cellulose. It may be preferred for this adsorption step to be carried out at acidic pH. When the adsorption step has been completed, the oxidised

cellulose onto which iron (III) has been adsorbed can be removed from the aqueous solution, and dried if so desired . It may be preferred within this embodiment to subject the Fe (III) -containing material to a post-treatment step with a base, in gaseous or dissolved form, to improve the bonding of the iron to the cellulose substrate. Suitable bases include, e.g., sodium carbonate , sodium hydroxide, potassium

carbonate, potassium hydroxide, NH3 , and NH40H.

Other methods are also possible. For example, it is possible to first add the iron to the cellulose, and then carry out the oxidation step . This is less preferred, however , as the presence of iron may interfere with the oxidation reaction. It is also possible to carry out the addition of iron and the oxidation reaction simultaneously, but this is also

considered less preferred.

In one embodiment of the present invention, a complex of Fe (III) and oxidised cellulose is prepared as follows:

Oxidised cellulose is contacted with an aqueous suspension comprising one or more Fe (III) oxide, Fe (III) hydroxide, and Fe ( III ) oxyhydroxide , followed by removal of water, to form a complex of oxidised cellulose with iron. The aqueous

suspension comprising one or more Fe (III ) oxide,

Fe (III) ydroxide, and Fe (III) oxyhydroxide, can suitably be obtained by adding a water soluble inorganic base, in solid form, or in the form of an aqueous solution, to a solution of an inorganic Fe (III) salt, e.g., a sulphate, nitrate, or chloride salt. In this way, a suspension will be obtained wherein the Fe ( III ) (hydr) oxide compounds have a relatively small particle size, which makes for a high dispersion of the Fe (III) (hydr) oxide compounds on the cellulose. The water soluble inorganic base is, e.g., selected from one or more of sodium carbonate, sodium hydroxide, potassium carbonate, potassium hydroxide, NH3, and NH40H . This method may be particularly suitable for oxidised cellulose with a particle size below 100 microns to ensure a good Fe (III) distribution.

In another embodiment of the present invention, a complex of Fe (III) and oxidised cellulose is prepared as follows:

Oxidised cellulose is contacted simultaneously with an aqueous solution of an inorganic Fe (III) salt, e.g., a sulphate, nitrate, or chloride salt, and an aqueous solution of a water soluble inorganic base , e.g., selected from one or more of sodium carbonate, sodium hydroxide , potassium

carbonate, potassium hydroxide , and NH40H, followed by removal of water . In this embodiment, one or more

Fe (III) oxide, Fe (III) hydroxide, and Fe (III) oxyhydroxide, are formed in the presence of the oxidised cellulose . This may lead to an improved distribution of the Fe (III) (hydr) oxide compound on the oxidised cellulose. This method may also be particularly suitable for oxidised cellulose with a particle size below 100 microns to ensure a good Fe(III) distribution.

In a further embodiment of the present invention, a complex of Fe (III) and oxidised cellulose is prepared as follows: In a first step, oxidised cellulose is contacted with an aqueous solution of an inorganic Fe(III) salt, e.g., a sulphate, nitrate, or chloride salt. If necessary, excess water is removed, e.g., by filtration. Preferrably, the resulting Fe ( II I ) -containing oxidised cellulose is dried. Then, in a second step, the Fe (III) -containing oxidised cellulose is contacted with an aqueous solution of a water soluble

inorganic base, e.g., selected from one or more of sodium carbonate, sodium hydroxide, potassium carbonate, and

potassium hydroxide, followed by removal of water. In this embodiment, one or more Fe (III) oxide, Fe (III) hydroxide, and Fe (III) oxyhydroxide , are formed after the Fe (III) salt has been adsorbed onto the oxidised cellulose. This may lead to an improved distribution of the Fe ( III ) (hydr) oxide compound on the oxidised cellulose, especially, where the oxidised cellulose has a particle size of at least 100 microns. It can of course also be applied on oxidised cellulose with a smaller particle size.

It is noted that in the context of the present specification the term "complex of Fe (III) and oxidised cellulose" does not place a limitation on the chemical relationship between the iron (III) and the oxidised cellulose, as long as the iron is (chemically or physically) bonded with the cellulose.

The adsorbent used in the present invention may comprise additional components to the complex of Fe (III) and oxidised cellulose. Examples of additional components are bonding agents. Suitable bonding agents may, e.g., be cellulose type materials. It is preferred for the adsorbent to be made up for at least 50 wt . % of the complex of Fe (III) and oxidised cellulose, in particular for at least 70 wt.%, more in particular for at least 80 wt.%, in some embodiments for at least 90 wt.%. The reason for this preference is that the complex of Fe(III) and oxidised cellulose is responsible for the phosphate adsorption. The presence of other components will increase the volume of the adsorbent without

contributing to the phosphate adsorption. Therefore, the amount of other components is preferably limited.

The particle size of the adsorbent used in the present invention can vary within wide ranges, e.g., in the range of 10 microns to 5 mm. In one embodiment, the particle size of the adsorbent is at least 100 microns, in particular at least 200 microns, especially where the material will be used in an adsorption column, as this minimum particle size will result in an improved performance of this embodiment. The maximum particle size preferably is at most 4 mm, in particular at most 2 mm. A particle size range of 0.2-2 mm is considered preferred. Within the context of the present specification, particle size refers to the Dv50 , which is the median

diameter of the particle size distribution, where 50% of the volume of particles in a sample has a diameter above the median particle diameter, and where 50% of the volume of particles in a sample has a diameter of at most the median particle diameter. In the process according to the invention, the phosphate containing water fraction is contacted with the adsorbent. This can be done by methods known in the art, including providing the adsorbent in a fixed bed, fluidised bed or moving bed, in a column, in a stirred reactor, or in any other way. The use of a column is preferred. Contacting conditions are not critical, and encompass a contacting temperature of 0-100°C, in particular 1-50°C, more in particular 1-30 °C . The pressure may vary between wide ranges, e.g. from 0.1 to 10 bar. Atmospheric pressure is generally suitable. Where a column is used, a velocity of 0.1-100 m/hour is generally suitable.

An effluent water fraction is withdrawn from the adsorbent. The phosphate content of the effluent water fraction is lower than the phosphate content of the starting phosphate- containing water fraction.

In one embodiment, the phosphate content of the effluent water fraction is less than 50% of the phosphate content of the starting phosphate-containing water fraction, in

particular less than 25%, more in particular less than 10%. In one embodiment, the phosphate content of the effluent water fraction is reduced to a value of less than 100 ppb, in particular less than 50 ppb, more in particular less than 20 ppb, or even less than 10 ppb. A process wherein the

phosphate content of the effluent water fraction is below 10 ppb is of particular interest, especially, but not limited to, the situation where the effluent water fraction is to be provided to a reverse osmosis process, as will be discussed in more detail below.

The phosphate content of the effluent water fraction is determined by the phosphate content of the starting water fraction, the carboxylate and iron content of the adsorbent, the space velocity, and the amount of adsorbent. It is within the scope of the skilled person to select these parameters so that an effluent with the desired phosphate content is obtained .

The effluent water fraction can be processed as desired. In one embodiment, the effluent water fraction is provided to a reverse osmosis step, where the effluent water fraction is treated to form a purified effluent water fraction and a contaminant fraction. The reverse osmosis step effects contaminant removal, in particular salt removal.

When the adsorbent has become saturated with phosphate, as can be seen from an increase in phosphate content of the effluent water fraction, the adsorbent can be regenerated if so desired.

Therefore, the present invention also pertains to a method as described above wherein the adsorbent is periodically

regenerated by a process comprising the steps of

- stopping the provision of phosphate-containing water fraction to the adsorbent

- contacting the adsorbent with an alkaline aqueous

regeneration solution,

- withdrawing the alkaline aqueous regeneration solution from the adsorbent, and

- resuming the provision of phosphate-containing water fraction to the regenerated adsorbent.

The adsorbent can be regenerated by contacting it with an alkaline aqueous regeneration solution, more specifically an aqueous solution with a pH above 11.5. The pH preferably is above 12. In general, the pH will not be above 14.

The nature of the alkaline compound in the aqueous alkaline solution is not critical. Alkali metal hydroxides are

generally preferred for reasons of availability, cost, and safety. The use of sodium hydroxide and/or potassium

hydroxide is considered preferred.

It may be preferred to use a solution comprising both an alkali metal hydroxide and a dissolved inorganic salt, in particular an alkali metal salt, e.g., NaCl or KC1. The presence of a salt has been found to improve regeneration efficiency. A salt concentration of 0.05 to 1 M/l may be mentioned as suitable.

Contacting the adsorbent with the aqueous alkaline

regeneration solution can be carried out using methods known in the art. Regeneration will be complete when the phosphate content of the regeneration solution does not further increase .

If so desired, it is possible to neutralise the adsorbent between the regeneration step and the restart of the

phosphate removal step. In this case, the adsorbent is contacted with a neutralizing solution between the step of withdrawing the alkaline aqueous regeneration solution from the adsorbent, and the step of resuming the provision of phosphate-containing water fraction to the regenerated adsorbent, the adsorbent is contacted with a neutralizing solutio .

A suitable neutralising solution is, e.g., a slightly acidic solution with a pH in the range of 4-6.

Various embodiments and preferences described herein can be combined, as will be evident to the skilled person, except where they are mutually exclusive.

The invention will be elucidated by the following examples, without being limited thereto or thereby.

Example 1 preparation of oxidised cellulose

An a-cellulose obtained from Sigma Aldrich with a particle size of 50 microns {further indicated as a-Ceil) was used as starting material.

The following oxidation methods were applied: Method 1 : Oxidation with hypochlorite/bromide

3.24 g cellulose was suspended in 500 ml water. After adjusting the pH to 11, lOOmg sodium bromide was added, followed by sodium hypochlorite. Subsequently, sodium

hypochlorite (15 % active chlorine) was added at once, the amount adjusted to obtain the desired degree of oxidation. The pH was maintained at pH 10 by addition of 0.5 M NaOH. When no further pH drop was observed, 100 mg NaBH< was added. After 60 minutes, 110 ml ethanoi absolute was added and the pH was adjusted to 7 by addition of 4 M sulphuric acid. The solid was isolated by centrifugation and washed repeatedly with ethanoi (70%) . The isolated material was dried in air.

Method 2: Oxidation with hypochlorite/bromide/TEMPQ

3.24 g was suspended in 500 ml water. After adjusting the pH to 11, lOOmg sodium bromide and 20 mg TEMPO (2,2,6, 6- tetramethylpiperidine-N-oxyl) were added. Subsequently, sodium hypochlorite (15 % active chlorine) was added all at once, the amount adjusted to obtain the desired degree of oxidation. The pH was kept at 11 during reaction by adding

0.5 M NaOH, After 60 minutes 1 ml ethanoi was added to quench the residual hypochlorite, followed by 100 mg NaBH4. After 60 minutes 110 ml ethanoi absolute was added and the pH was adjusted to 7 by addition of 4 M sulphuric acid. The solid was isolated by centrifugation and washed repeatedly with ethanoi (70%) . The isolated material was dried in air.

The properties of the various cellulose materials are present in Table 1 below. Table 1. properties of cellulose materials

Example 2: Preparation of complex of Iron (III) and cellulose A solution of 3.61 g iron (III) chloride hexahydrate (13 mmol) in 7 ml water, and a solution of 2.87 g Na2C03 (27 mmol ) were added simultaneously to 1.0 g cellulose (non- oxidised and oxidised samples) , suspended in 5ml water in the course of 5 minutes while stirring continuously. After stirring during 1 hour, the suspension was centrifuged. The pellet was washed several times with water and finally with ethanol (96%) . Then the solid was dried in vacuum at 70 °C ( ImmHg) .

The resulting material had an iron (III) content of 35 wt.%, expressed as metallic iron per gram of cellulose.

Example 3 : Performance of samples in phosphate removal from phosphate-containing water fractions - static conditions The phosphate adsorption capacity for the various samples was determined as follows: 1 g of the iro (III) on cellulose material obtained as described in Examples 1 and 2 was brought in a 30 ml column. Through the column 250 ml of a phosphate solution (0.47 g/L) was recirculated (5 ml/min) for a period of 18 hours at room temperature.

The adsorption capacity of Lewatit® FO 36 (Lanxess ) was also determined. Lewatit® FO 36 is a weakly basic macroporous monodisperse polystyrene-based ion exchange resin for the selective adsorption of oxoanions, which is doped with a nano-scaled film of iron oxide covering the inner surfaces of the pores of the polymer bead.

The results are summarized in Table 2.

Table 2. Phosphate adsorbing capacity of iro (III) cellulose complexes

From the data in table 2 it can be seen that the Fe(III) cellulose complex based on oxidised cellulose has a phosphate adsorption capacity which is improved by a factor of more than 5 as compared to an Fe (III) cellulose complex based on non- oxidised cellulose. The oxidised cellulose also has a higher phosphate adsorption capacity than Lewatit® FO 36.

Example 4 : Phosphate uptake by oxidised cellulose and

Lewatit® F036 - dynamic conditions

Kinetic experiments were performed as follow: 1.0 g of the sample to be tested was added to a solution containing 1.04 mg/L phosphate while stirring mechanically. From this

suspension 25 ml samples were taken at 1, 2, 5 , 15, 30, 60 and 90 minutes. Immediately after sampling, the sample was filtrated over a 0.2 mm cellulose acetate filter to remove present adsorbent. The phosphate concentration was determined using the phosphate molybdenum test. The. rate of phosphate adsorption onto the adsorbent is calculated from the decrease in phosphate concentration in the solution.

The results are provided in Table 3 below

Table 3 : Rate of phosphate adsorpt

As can be seen from Table 3, the material according to the invention shows the same kinetic profile as the standard Lewatit® FO 36, showing that the material according to the invention is suitable for commercial operation.

Example 5 : Regeneration

The starting material for the adsorbent regeneration was an of-cellulose sample oxidised in accordance with Method 1 in Example 1, with a degree of oxidation of 10% , which had been provided with iron in accordance with Example 2, and

saturated with phosphate in accordance with Example 3.

100ml 0.1M NaOH solution was flushed through 1.0 gram of the adsorbent at lOml/min for 1 hour. Afterwards, the pH and phosphate concentration of the NaOH solution was determined. After regeneration, the adsorbent was flushed with demi-- a er until neutral , after which another static phosphate removal test was performed to determine regeneration efficiency. The results are summarized in Table 4. Table 4;

¹ Possibly due to the phosphate already present in the starting cellulose or to experimental error.

It follows from this experiment that regeneration of these materials is possible.

Example 6: oxidation of pelletised cellulose

10 g of cellulose pellets (100% macrocrystalline cellulose, from IPC} were suspended in 40 ml tap water. 400mg NaBr was added, followed by 4 ml NaOCl (2, 4, 6 or 13% active C12, diluted from a 13% solution) . The pH was maintained at 9-10. Every 10-15 minutes an aliquot of NaOCl was added, until the final volume mentioned in the table below was reached. During the reaction, the pH drops gradually. Once all NaOCl was added, the pH was maintained at around 9 using 0.5 ml additions of NaOH (1M) . The pellets were washed with several portions of 100ml tap water and dried at 50°C .

Example 7 : preparation of complex of iron (III) and cellulose pellets

The oxidised and dried cellulose pellets obtained in Example 6 were soaked in 5.76 ml FeC13 solution (40% w/w/ , d = 1.26 g/ml) , diluted to 11.5 ml using demiwater for 30 minutes, after which they were dried at 40°C . The material was allowed to cool to room temperature and neutralised by addition of 5.02 ml NH40H (-27% NH3) , diluted to 11.5 ml using demiwater. The material was soaked for 30 min, after which it was dried at 0°C . The material was allowed to cool to room

temperature, after which it was washed with demiwater to remove fines and free iron. The final product had an iron (III) content of 10 wt . % .

The maximum adsorption capacity was determined via a static phosphate removal test analogous to the test described in Example 3.

Table 5:

Example 8: preparation of complex of iron (III) and cellulose using NH40H as base

180.5 g of iron (III) chloride hexahydrate was dissolved in 350 ml of water. 184.5 ml of NH40H (-27% NH3) was diluted with 385 ml of water and added to the iron (III) chloride solution while stirring vigorously. The iron oxide suspension was stirred for an hour, and then allowed to rest for an additional hour. 50 g of oxidised a-cellulose suspension was added to the iron suspension and the resulting mixture was stirred using a magnetic stirring bar overnight. The

suspension was centrifuged and the supernatant was discharged. The solid was resuspended in EtOH and centrifuged again. The resulting material had an Fe (III) content of 70 wt . % (calculated as g/kg) . It had a phosphate adsorption capacity of 53.75 g/kg.

Example 9: Phosphate removal

A column was filled with 3.19 gram of pelletised oxidised cellulose obtained in Example 6. A water fraction comprising 476 ppb of phosphate was provided to the unit under the conditions indicated in Table 7 :

Table 7:

The phosphate content of the effluent was determined at regular intervals. Figure 1 shows the adsorption curve. As can be seen from Figure 1, the material according to the invention is able to reduce the phosphate content from the level of 500 ppb to below the detection limit. After 500 bed volumes, the phosphate content of the effluent is still below 30 ppb. After move bed volumes, the phosphate content in the effluent starts to increase, due to increasing saturation of the bed.

Claims

1. Method for removing phosphate from a phosphate- containing water fraction comprising the steps of contacting a phosphate-containing water fraction with an adsorbent, the adsorbent comprising a complex of Fe (III) and oxidised cellulose, the oxidised cellulose having a carboxylate content of at least 200 microequxvalent carboxylate per gram, and withdrawing an effluent water fraction from the

adsorbent, wherein the complex of Fe (III) and oxidised cellulose has an Fe (III) content of at most 70 wt.%,

expressed as metallic iron, per gram of oxidised cellulose.

2. Method according to claim 1, wherein the phosphate- containing water fraction has a phosphate content of at least 20 ppb, in particular at least 50 ppb, more in particular at least 100 ppb, and/or a phosphate content below 1000 ppb, more specifically below 500 ppb.

3. Method according to any one of the preceding claims, wherein the oxidised cellulose has a carboxylate content of at least 300 peq/g, in particular at least 400 peq/g and/or at most 2000 peq/g, in particular at most 1000 peq/g.

4. Method according to any one of the preceding claims, wherein the complex of Fe (III) and oxidised cellulose has an Fe (III) content of at least 1 wt.%, expressed as metallic iron, per gram of oxidised cellulose, in particular at least 5 wt.%, more in particular at least 10 wt.%, and/or at most 50 wt.%, more in particular at most 30 wt.%, or even at most 20 wt.%.

5. Method according to any one of the preceding claims, wherein the phosphate containing water fraction is contacted with the adsorbent in a fixed bed, fluidised bed or moving bed, in a column, or in a stirred reactor, preferably a column .

6. Method according to any one of the preceding claims, wherein the phosphate content of the effluent water fraction is less than 50% of the phosphate content of the starting phosphate-containing water fraction, in particular less than 25%, more in particular less than 10%.

7. Method according to any one of the preceding claims, wherein the phosphate content of the effluent water fraction is reduced to a value of less than 100 ppb, in particular less than 50 ppb, more in particular less than 20 ppb, or even less than 10 ppb, in particular less than 10 ppb.

8. Method according to any one of the preceding claims, wherein the effluent water fraction is provided to a reverse osmosis step.

9. Method according to any one of the preceding claims wherein the adsorbent is periodically regenerated by a process comprising the steps of

- contacting the adsorbent with an alkaline aqueous

regeneration solution,

10. Method according to claim 9, wherein the alkaline aqueous regeneration solution has a pH of at least 11.5.

11. Method according to claim 9 or 10, wherein between the step of withdrawing the alkaline aqueous regeneration

solution from the adsorbent, and the step of resuming the provision of phosphate-containing water fraction to the regenerated adsorbent, the adsorbent is contacted with a neutralizing solution.

12. Method according to any one of the preceding claims, wherein the complex of Fe (III) and oxidised cellulose is obtained by contacting oxidised cellulose with an aqueous suspension comprising one or more Fe ( III ) oxide,

Fe (III) hydroxide, and Fe (III) oxyhydroxide , followed by removal of water, the aqueous suspension comprising one or more of Fe (III) oxide, Fe (III) hydroxide, and Fe (III)

oxyhydroxide, preferably being obtained by adding a water soluble inorganic base, in solid form, or in the form of an aqueous solution, to a solution of an inorganic Fe (III) salt.

13. Method according to any one of claims 1-11, wherein the complex of Fe(III) and oxidised cellulose is obtained by contacting oxidised cellulose simultaneously with an aqueous solution of an inorganic Fe ( III ) salt and an aqueous solution of a water soluble inorganic base, followed by removal of water .

14. Method according to any one of claims 1-11, wherein the complex of Fe(III) and oxidised cellulose is obtained by in a first step contacting oxidised cellulose with an aqueous solution of an inorganic Fe ( III ) salt, and in a second step contacting the Fe ( III ) -containing oxidised cellulose with an aqueous solution of a water soluble inorganic base, followed by removal of water.

15. Method according to any one of the preceding claims, wherein the adsorbent has a particle size in the range of 10 microns to 5 mm.