WO2003050043A2

WO2003050043A2 - Water treatment using ferrate

Info

Publication number: WO2003050043A2
Application number: PCT/GB2002/005663
Authority: WO
Inventors: Jiaqian Jiang
Original assignee: University of Surrey
Current assignee: University of Surrey
Priority date: 2001-12-13
Filing date: 2002-12-13
Publication date: 2003-06-19
Anticipated expiration: 2004-06-13
Also published as: GB0415022D0; WO2003050043A3; AU2002350959A8; GB0129773D0; GB2398777A; GB2398777B; AU2002350959A1

Abstract

A formulation for use in the treatment of water and wastewater to remove impurities and harmful micro-organisms comprises a ferrate [Fe(VI)] ion combined with at least one chemical species, preferably a polymerised aluminium species. The ferrate and polymerised aluminium may be combined in a molar ratio ferrate (as Fe) to Al of between 1:2 and 1:4. The composite chemical enables treatment to be carried out at close to neutral pH, typically in the range 6.0 to 8.0, with lower doses compared to the use of ferrate ion alone. In this way, the coagulating/disinfecting properties can be optimised for water treatment.

Description

IMPROVEMENTS IN OR RELATING TO WATER AND WASTEWATER TREATMENT

This invention concerns improvements in or relating to water and wastewater treatment. The invention has particular, but not exclusive, application for the removal of impurities and/or harmful micro-organisms from water and wastewater to be used in the drinking water supply. More especially, the present invention concerns a chemical formulation for use in the treatment of water and wastewater and to a method for producing a compound employed in the formulation. For convenience, the term "water" will be used hereinafter to include water from any source including wastewater and is to be construed accordingly.

Existing water treatment processes typically employ coagulation and disinfection for removing impurities and harmful micro-organisms from drinking water. Coagulation destabilizes colloidal impurities and transfers small particles into large aggregates and adsorbs dissolved organic materials onto the aggregates, which can then be removed by sedimentation and filtration. Disinfection in water treatment is designed to kill the harmful micro-organisms (e.g. bacteria and viruses) and to control/remove the odour precursors.

A wide range of coagulants and disinfectants can be used for water and wastewater treatment. The most common coagulants used include ferric sulphate, aluminium sulphate, and ferric chloride, and the oxidants/disinfectants used are chlorine, sodium hypochlorite, chlorine dioxide, and ozone. As water pollution increases and the standards of drinking water supply and wastewater discharge become stricter, there is a need for more cost-effective water treatment chemicals in order to achieve higher treated water qualities. Moreover, as the coagulation and disinfection are usually carried out in separate unit processes, there is a need to develop more efficient, lower cost processes for producing a water supply of an acceptable quality for drinking.

The present invention has been made from a consideration of the foregoing and seeks to provide water treatment formulations capable of disinfecting micro-organisms, partially degrading and oxidising organic and inorganic impurities, and removing colloidal/suspended paniculate materials and heavy metals whereby the coagulation and disinfection processes may be combined.

According to a first aspect of the invention, there is provided a formulation for use in the treatment of water to remove impurities and harmful micro-organisms comprising a ferrate [Fe(NI)] ion combined with at least one chemical species whereby the coagulating/disinfecting properties of the formulation are optimised compared to the use of the ferrate ion alone.

The present invention is based on the recognition that the disinfection and coagulation processes can be combined in a single process using ferrate ions and we have surprisingly found that a cost-effective formulation for water treatment can be obtained by combining ferrate ions with another chemical species that allows the amount of ferrate ions used to be reduced without any adverse effect on the water treatment. Furthermore, we have found that such water treatment can be carried out at pH close to neutral, typically in the range pH 6.0 to 8.0. In contrast existing water treatments can require a pH as low as 4.5 to achieve effective coagulation for removing colour-causing organic matter.

The ferrate ion contains hexavalent (NI) iron and has the molecular formula

The ferrate ion is a very strong oxidant under acidic conditions (see Table 1 below) while itself being reduced to ferric [Fe(III)] ions or ferric hydroxide that are good coagulants. As a result, the disinfection and coagulation processes employed for water treatment can be achieved in a single dosing and mixing process by the use of ferrate ions.

Table 1.

Comparison of the Oxidation and Reduction Potential of the Ferrate ion with the Oxidants/Disinfectants commonly used in Water and Wastewater Treatment

Despite the potential advantages of the ferrate ion for water treatment due to its unique properties for combining the disinfection and coagulation processes, we are not aware of any commercial exploitation of the use of ferrate ions for water and wastewater treatment. One reason for this is believed to be due to the relatively high cost of producing the ferrate ions which has made its use in a single treatment process uneconomic compared to the use of other chemicals in separate disinfection and coagulation treatment processes.

Thus, it is known to produce potassium ferrate using a wet oxidation method that involves the formation of sodium ferrate by reacting ferric chloride with sodium hypochlorite (NaCIO) in the presence of sodium hydroxide, and then adding potassium hydroxide to precipitate the potassium ferrate from the solution mixtures. The basic chemical reactions are shown as follows:

Fe³* + 30H → Fe(OH)₃ (1)

2Fe(OH)₃ + 3NaClO + 4NaOH → 2 Na₂FeO_< + 3NaCl + 5H_zO (2) Na₂FeO_< +2KOH → K₂FeO_< +2NaOH (3)

Using hypochlorite in the preparation of the ferrate incurs toxic side effects resulting from the use of excess gaseous chlorine in a caustic soda solution to produce the hypochlorite. Therefore, the preparation steps and conditions need to be strictly controlled. In addition to this, in order to prepare a high purity ferrate salt, several purification procedures are required which increases the cost of the ferrate with the result that water treatment with ferrate alone is very costly and uneconomical compared to existing methods of treatment.

We have found that combining a ferrate with another chemical species especially a cation, enables the advantages of single dosing with ferrate to be employed at lower running costs without sacrificing the effectiveness of the ferrate. Moreover, for certain chemical species, in particular polymerised aluminium species, the performance of the formulation may be enhanced compared to treatment with the ferrate alone. The preferred polymerised aluminium species can be made by partial hydrolysis of acid aluminium salt solution using a specific reactor. The nature of the polymeric species formed depends on various factors such as the concentration of Al, the basic ratio r, defined as the molar ratio of the hydroxide ions to metal ions, base concentration, base addition rate, and the hydrolysis duration of Al solution (ageing time) and ageing temperature. The most important parameters that govern the nature of the species are the basic ratio and the ageing temperature and time. Preferably, the ferrate and aluminium are combined in the molar ratio ferrate (as Fe) to Al ranging between 1:0.5 and 1:8 with a ratio of between 1 :2 and 1 :4 being especially preferred.

Although the combination of ferrate and polymerised aluminium species is the preferred formulation for water treatment according to the invention, other cationic ions (e.g. , Mn and Si) can also be used to partially replace the ferrate in the resulting composite chemical with the same enhancement of the overall treatment performance of the formulation.

We have also found that ferrate can be produced by an alternative method that reduces the production cost of the ferrate and results in further cost savings for the manufacture and use of the invented formulation for the treatment of water according to the first aspect of the invention.

Thus according to a second aspect of the invention, there is provided a method of manufacturing a ferrate [Fe(NI)] comprising the oxidation of ferrous [Fe(II)] and/or ferric ions [Fe(III] to ferrate ions under alkaline conditions in the presence of monoperoxosulphate. Preferably, a molar ratio of monoperoxosulphate to iron is in the range between 0.4: 1 and 4: 1 with a molar ratio in the range between 0.8: 1 and 2.5: 1 being especially preferred.

The method preferably includes the step of mixing the reactants at low temperature, preferably under 10°C, and separating the resulting ferrate solution from the precipitates, for example by centrifuging.

According to a third aspect of the invention, there is provided a method of making a composite formulation for water treatment according to the first aspect of the invention by mixing a ferrate solution with a chemical species to produce the desired composite formulation.

Preferably, the ferrate solution is produced by the method according to the second aspect of the invention. Alternatively, the ferrate solution may be produced by oxidising ferrous and/or ferric ions with any other suitable oxidising agent, for example potassium permanganate.

According to a fourth aspect of the invention, there is provided a method of water treatment to remove impurities and kill harmful micro-organisms by the use of a composite chemical formulation including ferrate [Fe(NI)] ions and another chemical species for carrying out both disinfection and coagulation in a singe process.

The preparation of the new composite water treatment chemical formulation according to the present invention essentially involves mixing a polymerised Al species with a ferrate at an optimal ratio of Al to ferrate, and a subsequent solidification procedure to prepare the composite chemical as a solid product.

The polymerised Al species may be prepared using an established method. The ferrate may be prepared by any known method but is preferably using a new method developed through this invention. The preparation of the polymerised Al species and the ferrate and the production of the composite chemical therefrom will now be described.

1.1 Preparation of the polymerised Al species

Aluminium polymerised species were made by partial hydrolysis of 100 ml of aluminium sulphate solution (1 M as Al) with 100 ml base (2.0 M solution of sodium bicarbonate), which gives the final basic ratio of 2 (basic ratio = the molar ratio of hydroxide to Al) , and the final Al concentration was 0.5 M. The base addition rate was 1 ml/min and the reaction temperature was controlled at 60 °C. An extra 1-hour ageing was required in order to generate the polymerised Al species.

1.2. Preparation of the ferrate using the new method

The new method of the preparation of ferrate involves the use of an alternative oxidant to oxidise the ferric ion to form the ferrate ion under alkaline conditions. The method generally comprises mixing a given amount of ferric sulphate working solution with a given amount of the desired oxidant, potassium monoperoxosulphate, and then adding the mixture into a pre-chilled 6M KOH solution and controlling the reaction temperature below 10° C using an ice bath.

The effect of varying the amount of oxidant on the resulting ferrate was investigated by reacting a given amount of a known ferric sulphate working solution with different amounts of a known potassium monoperoxosulphate solution. The ferric sulphate working solution (1M as Fe) was produced by dissolving 245.6 g of pentahydrolate ferric sulphate in de-ionised water and making the final volume of the solution up to 1 litre. This gives a ferric iron concentration of 1 mole Fe/1. The resulting solution was stored in a fridge. A 6M KOH solution was prepared by dissolving 336.66g KOH into 1 litre of de-ionised water and the resulting solution was kept in the fridge.

20ml portions of the ferric iron working solution (0.02 mol Fe) were mixed with different amounts of potassium monoperoxosulphate to give desired molar ratios of monoperoxosulphate to iron in the range between 0.4:1 and 4:1. The resulting mixtures were then slowly added into 50ml of pre-chilled 6M KOH under rapid agitation and keeping the reaction temperature below 10° C. The characteristic purple colour of the ferrate developed when the solutions were mixed and, on completion of the mixing, the liquid ferrate was immediately separated from the precipitates by centrifuging at 4500 rpm for 14 min.

The ferrate supernatant obtained was analysed immediately using a spectroscopy method by dilution of the ferrate sample and measuring the absorbance at its characteristic wavelength of 504 nm and subsequently converting this to the ferrate concentration using a conversion coefficient, ε = 1060 cm 'mol^-1 1. The ferrate concentration was also measured using a chromite method described by Schreyer J.M. , Thompson G.W. and Ockerman L.T. , 1950. Oxidation of chromium(III) with potassium ferrate(IN) , Anal. Chem. , 22(11) , 1426-27.

The results are shown in Table 2 below from which it can be seen that the amount of ferrate produced is significantly increased for a ratio of monoperoxosulphate to iron of between 0.8: 1 and 2.5:1. Table 2.

Effect of oxidant dose on the concentration of the resulting ferrate

* Dilution factor = 120

1.3 Preparation of the water treatment formulation.

A mix of the polymerised Al species (see 1.1 above) with the ferrate (see 1.2 above) was conducted by slowly adding a given amount of

10 polymerised Al into a given amount of ferrate solution to provide a desired molar ratio of Fe:Al. The resulting liquid composite water treatment chemical formulation was then converted to a solid form by a pre-freeze step and then dried by a freezer dryer (Heto Freezer Dryer-3, Heto Corp. , Denmark) at -52°C. The resulting solid product of the

15 composite water treatment chemical can be kept at room temperature without causing the degradation and is soluble when added to water for treatment to produce the ferrate ion. The ferrate and aluminium contents in the final product can vary based on the molar ratio of ferrate (as Fe) to Al, ranging between 1 :0.5 and 1 :8.

20 The invention will now be described in more detail with reference to the following examples and accompanying drawings wherein :-

Figures 1 to 3 are graphs comparing the performance of the composite chemical according to the present invention with ferric sulphate for treating water at a coagulation pH of 4.5, 6.0 and 8.0 respectively.

Example 1

3.2 g of the solid composite water treatment chemical formulation with the molar ratio of Ferrate (as Fe) to Al of 1:2, and the total ferrate content and Al content are 0.01 mol as Fe and 0.02 mol as Al, respectively, and the potassium ferrate content of 62.0% w/w was prepared as follows :-

5 g of potassium monoperoxosulphate was mixed with 20ml of 1M iron working solution (0.02 mol Fe) . The mixture was then slowly added into an ice-surrounded reactor, containing 50ml pre-chilled 6M KOH, under rapid agitation. The characteristic ferrate purple colour developed when the solutions were mixing. The reaction temperature was controlled to be less than 10 °C. After finishing mixing, the liquid ferrate was immediately separated from the precipitates by centrifuging at 4500 rpm for 14 min. The ferrate solution was then slowly mixed with 25 ml pre- prepared polymerised aluminium ([Al] = 0.5 M, and basic ratio = 2) . The resulting liquid chemical was then converted to a solid form by a pre- freeze step and then dried by a freezer dryer (Heto Freezer Dryer-3, Heto Corp. , Denmark) at -52°C. The prepared composite water treatment chemical was then ready for use. Properties of the prepared composite water treatment chemical formulation:

Potassium ferrate content (w/w) = 62.0 The molar ratio of Ferrate (as Fe) : Al = 1 :2

Example 2

The composite chemical formulation prepared by the method described in Example 1 was evaluated as a disinfectant in comparison with hypochlorite and hypochlorite plus ferric sulphate for killing the total coliform and faecal coliform using a lake water as the test water.

The results are shown in Table 3 below from which it can be seen that the composite chemical formulation according to the invention demonstrated a greater disinfection capability in comparison with other disinfectants. In addition, a very low dose of the composite chemical formulation was sufficient to achieve 100% killing of the targeted bacteria.

Table 3.

Comparative disinfection performance of various disinfectants for achieving 100% killing* of Bacteria

* 100% killing = non-detectable (ND) f The tests were conducted under the condition of 30 minutes contacting time at 3 disinfection pH values, and the total and faecal coliforms were measured using a standard membrane filtration method. The original bacteria number concentrations were as:

Total Coliform = 2.1xl0⁴ and, Fecal Coliform = 8.0xl0³. Example 3

The composite chemical formulation prepared by the methods described in Example 1 was also evaluated as a disinfectant in comparison with the hypochlorite for killing the Escherichia coliform (E. coli) using a synthetic water as the test water. The tests were conducted under the condition of 30 minute contacting time, and E. coli. were measured using a standard membrane filtration method.

The results are shown in Tables 4(a), 4(b) from which it can be seen that the composite chemical formulation demonstrated a greater disinfection capability for treatments at two disinfection pH levels, 5.5 and 7.5 respectively. In particular, smaller doses of the composite chemical formulation according to the invention were sufficient to kill E. coli at a non-detectable level compared to hypochlorite.

Table 4(a).

The remaining E. coli. number concentrations treated by hypochlorite

Table 4(b).

The remaining E. coli. number concentrations treated by prepared composite chemical formulation

ND = non-detectable.

Example 4

The composite chemical formulation prepared by the methods described in Example 1 was further evaluated as a coagulant for treating a model coloured water in comparison with potassium ferrate, ferric sulphate and aluminium sulphate.

The results are shown in Table 5 from which it can be seen that the composite chemical demonstrated a greater coagulation capability for the removal of humic acid (colour) from the water at a relative low dose Table 5

The comparative coagulation performance* for the removal of humic acid

♦The doses of different coagulants were 0.2 inM as the total metal (either Al or Fe or Al + Fe), and the coagulation pH was maintained at 5.5. t The UN254 absorbance of the model water was 14.8 m ¹.

Example 5

The effectiveness of the composite chemical formulation prepared by the methods described in Example 1 as a coagulant for treating a model coloured water was further evaluated in comparison with ferric sulphate for the removal of fulvic acid (colour) at different pH values and different dosing levels. As will be appreciated, fulvic acid is more difficult to remove with coagulation than humic acid due to the smaller molecular size of fulvic acid compared to humic acid.

The results are shown in Figures 1, 2 and 3. The model water contained 15 mg L^"1 as fulvic acid and UN₂₅ absorbance of the model water was 35 m^" As can be seen, for a given dose at each coagulation pH, the composite chemical demonstrated a greater coagulation capability for the removal of fulvic acid (colour) from the water compared to ferric sulphate. Moreover, the performance of the composite chemical is generally maintained when the coagulation pH is increased from pH 4.5 to pH 8 whereas the performance of the ferric sulphate at the same coagulation pH decreases with a significant reduction in performance at coagulation pH 8.

At coagulation pH 4.5 and pH 6.0, the performance of the composite chemical is better than ferric sulphate (FS) in terms of UN₂₅₄ removal (typically 10 to 20% more UN₂₅₄ removal) at each dose level. At coagulation pH 8, the performance of the composite chemical is significantly better than ferric sulphate (FS) in terms of UN₂₅ removal (typically 30 to 40% more UN₂₅ removal) at each dose level. In particular, it can be seen that there is a marked reduction in the effectiveness of ferric sulphate for the removal of UN₂₅₄ at the higher coagulation pH compared to the composite chemical which retains its performance over the coagulation pH range.

The superior performance of the composite chemical for removing FA at coagulation pH 6 and 8 is especially beneficial since pH in natural surface waters are normally in this range (between pH 6 and pH 8) . As a result, maximum FA removal may be achieved when using the composite chemical of the present invention at the pH levels occurring in natural surface waters. In this way, adjustment of the pH level below 6.0 as typically required when using other chemical treatments such as ferric sulphate to achieve maximum FA removal may be avoided. In addition, using the composite chemical to remove FA requires less dosage compared to existing chemical treatments to achieve maximum FA removal. It will be appreciated that carrying out FA removal at the naturally occurring pH level and at lower dose levels of the added chemical gives extra advantages when using the invented composite chemical for drinking water treatment.

The foregoing examples demonstrate that the new composite chemical formulation according to the present invention in which polymerised Al species are combined with ferrate has superior performance for water treatment in a single process compared to the use of chlorine (hypochlorite) disinfectant and ferric/aluminium coagulant in separate processes.

Moreover, the formulation in which ferrate [Fe(ND] is partially replaced by polymerised Al species performs as well as ferrate alone but at lower doses and thus lower running cost. In addition, using the formulation overcomes the problems of disinfection by-products arising from the use of the existing disinfectants such as chlorine, hypochlorite and ozone.

It will be apparent that the present invention provides a safe and cost effective chemical formulation which is stable in a solid form for use as a water and wastewater treatment which has the dual functions of oxidation and coagulation and can be used to kill effectively harmful microorganisms and remove heavy metals, synthetic organic matter and other contaminants without causing any toxic side effects.

The present invention also provides a new method to prepare ferrate [Fe(NI)] under a more environmental friendly condition compared to the use of chlorination in the conventional method. Consequently, the new method simplifies the preparation procedures and reduces the preparation cost of the ferrate. It will be understood that the above-described examples are intended to illustrate the invention and that features of the examples may be used separately or in combination with any other feature of the same or different examples. Moreover, while the examples are believed to represent the best means currently known to the applicant for putting the invention into practice, it will be appreciated that the invention is not limited thereto and that various modifications and improvements can be made within the spirit and scope of the claims.

Claims

1. A formulation for use in the treatment of water to remove impurities and harmful micro-organisms comprising a ferrate [Fe(ND] ion combined with at least one chemical species whereby the coagulating/disinfecting properties of the formulation can be optimised compared to the use of the ferrate ion alone.

2. A formulation according to claim 1 wherein the chemical species is a cation selected from the group comprising aluminium, manganese and silicon species.

3. A formulation according to claim 2 wherein the chemical species is a polymerised aluminium species.

4. A formulation according to claim 3 wherein the ferrate and polymerised aluminium are combined in a molar ratio ferrate (as Fe) to Al ranging between 1:0.5 and 1 :8.

5. A formulation according to claim 4 wherein the ferrate and polymerised aluminium are combined in a molar ratio ferrate (as Fe) to Al of between 1 :2 and 1:4.

6. A formulation according to any one of the preceding claims in the form of a solid for adding to water to be treated.

7. A method of making a composite formulation for water treatment according to any one of the preceding claims by mixing a ferrate solution with a chemical species to produce the desired composite formulation.

8. A method according to claim 7 wherein the ferrate solution is prepared by the oxidation of ferrous and/or ferric ions to ferrate ions under alkaline conditions in the presence of monoperoxosulphate.

9. A method according to claim 8 wherein a molar ratio of monoperoxosulphate to iron in the range between 0.4: 1 and 4:1 is employed.

10. A method according to claim 9 wherein the molar ratio is in the range between 0.8:1 and 2.5:1.

11. A method according to any one of claims 8 to 10 wherein the step of mixing the reactants is carried out at low temperature.

12. A method according to claim 11 wherein the low temperature is below 10°C.

13. A method according to any one of claims 8 to 12 including the step of separating the resulting ferrate solution from the precipitates.

14. A method of water treatment to remove impurities and kill harmful micro-organisms by the use of a composite chemical formulation according to any one of claims 1 to 6.

15. A method according to claim 14 wherein the water treatment is carried out at a pH in the range 6.0 to 8.0.

16. A method according to claim 15 wherein the water treatment is carried out at close to neutral pH.

17. A formulation for use in the treatment of water to remove impurities and harmful micro-organisms comprising a ferrate [Fe(NI)] ion combined with a polymerised aluminium species.

18. A formulation according to claim 17 wherein the formulation is employed at a pH of 6.0 to 8.0.

19. A formulation according to claim 18 wherein the formulation is employed at close to neutral pH.