Field of invention

The present invention relates to a method for the removal of contaminations from water by using chitosan to form precipitations with the contaminations, and then remove the formed precipitations by applying an electric field generated with direct current.

Background of the invention

Contaminations in water for use in the preparation of food products for human consumption represent a steadily increasing problem because of the increased use of pesticides and other chemicals that accumulate in the environment. Water can contain toxic compounds such as salts of heavy metals, water soluble organic and inorganic compounds, all of which must be eliminated before making use of the water.

Prior art

US 5480558 ("Electric enhancement of adsorbent flotation separation") relates to removing PCB from earth by a washing process. Contaminated earth is suspended in water in a tank, gas is bubbled through the slurry and together with the

contaminations form a foamed top layer.

US 5770090 ("Method for recovery of heavy metal from waste water") relates to a method for removing heavy metals, in particular chromium, from waste water.

Muzzarelli et. al: "Chitosan as a tool for the purification of waters"; Chitin Nat. Technol., [Proc. Int. Conf. Chitin Chitosan], 3rd (1986), Meeting Date 1985, pp. 551 -554. CODEN: 55MHAB, has performed a scientific investigation of chitosan as an adsorbent for different chemical compounds.

CN 101508503 (abstract) (Nantong University, 2009.08.19) discloses purification of sewage water by contacting the water with a solid adsorbent.

The use of chitosan in water cleaning has been known for long. In water purification the chitosan will absorb the contaminations and form precipitations which will partly sediment into a solid bottom layer, partly remain suspended in the processed water. The object of the present invention is to remove the precipitated, and particularly the suspended, substances in an efficient way by the use of an electric field generated with direct current. The method of the present invention can be performed by the use of a simple apparatus. The level of heavy metals and other contaminations remaining in the cleaned water will be far below the values set by health authorities.

Summary of the invention

The present invention relates to a method for removing contaminations from water. The method is characterised in that at least one absorbent which absorb the contaminations is mixed into the water, then the water comprising the absorbent is exposed to a direct current electric field between two or optionally more electrodes having a positive and negative charge, respectively, in a period of time sufficient to obtain that the absorbent with absorbed contaminations are deposited onto the positively charged electrode, and at the end this electrode with deposited substances is lifted out of the water, and the deposited substances are removed.

In an embodiment of the present invention a first absorbent and a second absorbent is used, wherein the first absorbent comprises chitosan and the second absorbent comprises activated carbon.

The method of the present invention can be performed both batchwise and continuously.

Brief description of figures

Fig 1 shows schematically a suitable vessel for use by the method of the invention; the vessel may also be provided with a heating jacket for temperature control.

Fig 2 shows the same vessel provided with a stirring device.

Fig 3 shows the vessel provided with an electrode.

Fig 4 shows the vessel with the anode lifted out for cleaning.

Detailed description of the invention

The present invention relates to a method for the purification of water by the use of at least one absorbent which form precipitations together with the

contaminations, and then remove all precipitations by using an electric field generated with direct current.

The content of foreign and potentially health hazardous substances in the purified water will be at a level well below the threshold values accepted or recommended by the health authorities.

By the present method any type of water of water can be purified, such as fresh water, seawater or brackish water.

In the purification process it is used at least one absorbent, preferably a first absorbent and a second absorbent.

Chitosan is used as the first absorbent, optionally together with carrageenan, alginates or condroitinglycol, or mixtures thereof.

The preparation of chitosan is well known and described in detail in the literature, e.g. in EP 1 ,412,391 B l . Chitosan may be prepared from chitin which can be found in the shell of marine shellfishes. Chitin is a natural polymer containing acyl moieties (-CH3-CO-). These can be converted to a desired extent by treating the chitin with alkaline water and thus obtain chitosan. Depending on the conditions when preparing chitosan. a desired degree of the acetylation can be obtained; that is to say that not ail acyl groups will necessarily disappear. In this way it is obtained a polymer chain consisting of randomly distributed chitosan and chitin monomers. The fraction of chitin is typically below 0.25. Chitin is insoluble in water, while chitosan is water soluble at acidic conditions, i.e. at pH below 7. Depending on the conditions of preparation the polymer chain will get different lengths. Therefore, chitosan is often characterized by the viscosity of an aqueous solution of chitosan. Typically, 1 percent by weight of chitosan is dissolved in 1 percent by weight of acetic acid, and the intrinsic viscosity ([η]) of the solution is determined. From this follows that chitosan may have different properties depending among others on the degree of the

acetylation, the distribution of chitin monomer units, chain lengths, cleavage products, and similar.

In the present invention all commercially available chitosan types may be used, also salts and glucose amides of chitosan, and hydrolyzed chitosan. The chitosan may have a degree of acetylation from high to low, and the whole range of viscosities.

In principle the chitosan could be mixed directly into the water, but since chitosan has poor water solubility at pH above 7, it is more suitable to use a chitosan- containing "purification" liquid. A typical purification liquid is prepared by dissolving chitosan in acidic water. As an acid all types of suitable acids may be used, preferably hydrochloric acid, acetic acid or hydroxyl acetic acid, most preferred hydroxyl acetic acid (glycolic acid). The purification liquid must have an acidic pH to keep the chitosan dissolved, i.e. pH must be below 6.5-7.

The purification liquid may be of various compositions and still provide acceptable results. As the purification liquid may be used an aqueous solution containing from about 0.1 percent by weight to about 10 percent by weight of chitosan and a strong acid in a sufficient amount of to keep the chitosan dissolved. In principle there is no lower limit for the content of chitosan in the purification liquid; however, for an acceptable effect the amount should be at least 0.1 percent by weight. The upper limit is determined by the solubility of chitosan in the purification liquid. Preferably the purification liquid comprises about 1 percent by weight of chitosan and about 1 percent by weight of an acid, such as glycolic acid.

Practical experimentations have shown that the efficiency of the "purification" liquid can be improved further if the solution in addition to chitosan also contains carrageenan and one or more of silica, bentonite, lime, coral sand, diatomaceous earth, or ion exchangers, or a mixture thereof. A particularly suitable purification liquid contains in addition to about 1 percent by weight of chitosan and about 1 percent by weight of an acid, also about 0.5 percent by weight of carrageenan and about 0.5 percent by weight of silica.

As soon as the first absorbent has been suspended in the water, the second absorbent may be added. As the second absorbent is preferably used activated carbon as particles, and preferably as powder, optionally together with bentonite, diatomaceous earth, lime, calcium carbonate, coral sand or ion exchangers, or a mixture thereof.

The addition of activated carbon has as a result that chitosan with absorbed substances coalesce into greater particles which can easily be removed. This reveals a surprising synergistic effect between chitosan and activated carbon. The amount of activated carbon used in the purification is not critical; but should be kept as low as possible for practical and economy reasons.

The above described purification liquid containing chitosan is mixed into the water followed by activated carbon. Another option is to add activated carbon to the purification liquid before this being added to the water. The invention is not restricted to any certain sequence of adding the absorbents.

During the addition of the first and the second absorbents the water must be stirred vigorously or have a turbulent flow in order to obtain that the absorbents are brought into an intimate contact with the contaminations. The contaminations will thereby be attached to the absorbents and form particles which can sediment and form a solid bottom layer. This bottom layer may be drawn off, or removed by the use of a filter or by mechanical means, depending o the circumstances.

When a possible formed bottom layer has been removed, the remaining water will contain a minor amount of suspended microparticles. These can be removed in a traditional way by filtering or centrifugation. However, it has surprisingly been found that the suspended substances can be removed in a more simple way by exposing the water to an electric field generated with direct current. In fact, the suspended microparticles will carry a negative charge and will thus migrate in such an electric field towards the positively charged electrode and attach thereto. With this method all contaminating substances in the water will be removed.

If the water is heavily contaminated, it may be appropriate first to remove a possible bottom layer by mechanical means, and then remove the remaining suspended m icropartic l es by the use of an electric field . I ndeed, the removal of precipitated particles by the use of an electric field may start at any stage of the purification process as soon as the mixing procedure is ended, that is to say as soon as the purification liquid has been completly mixed into the water. As an option, the total amount of precipitated substances can be removed by the use of an electric field, omitting any mechanical removal . Embodiments of the invention depend on the level of

contaminations in the water.

By the method of the invention the employed reactor vessel can be provided with an assembled negatively charged electrode and an positively charged electrode. Analogously to electrolysis the negatively charged electrode will be termed cathode, and the positively charged electrode will be termed anode. The jacket of the vessel may be the negative electrode, or a separate negative electrode may be used. An anode is immersed into the water in the center of the vessel. Between the electrodes is applied a direct current voltage typically in the range of 0.1 to 500 volts. Prior to exposing the water to an electric field a salt solution may be added to the water in an amount of about 0.1 % by weigt of the weight of the water to make the water electrical conductive. The salt used may be sodium chloride or ferric chloride. The salt solution may be added at any stage of the process. Such an addition may be necessary for fresh water, while such an addition is unnecessary for seawater.

The water containing suspended microparticles is exposed to the electric field in a period of time from about 10 seconds to a number of hours, depending on the circumstances.

The electrodes are composed of an electrical conductive material, such as a metal alloy. The anode may be of any suitable shape, such as one or more rods, plates, wire nets. etc. The cathode may be the jacket of the vessel or a separate electrode.

Chitosan is negatively charged, and as a consequence chitosan with absorbed substances will have a negative surface charge and be attracted to the anode on which the chitosan-containing substances will deposit. When chitosan containing absorbed contaminations has deposited on the anode forming a layer of appropriate thickness, the anode is lifted out of the vessel and cleaned. Deposited substances may adhere so firmly to the anode that mechanical means must be used to remove it.

The collected waste material must be disposed of in an approved depot for environmentally hazardous materials. All contaminated materials separated in the process, such as a bottom layer which is drawn off or removed by filtering or by mechanical means, can be subjected to evaporation until dryness. By the evaporation all contaminatins will remain bound to the chitosan. The chitosan will function in this way up to about 300 °C. The evaporation method may be conducted up to a

temperature of about 200 °C. At the end of the evaporation the only remainings are solid substances which are concentrated contaminations of a smallest possible volum.

The method will now be explained in more detail by reference to the figures.

A suitable vessel for use in the present method is shown in figure 1 . The vessel may be of any suitable size and may be provided with an outer heating jacket to maintain a constant temperature in the water if necessary. To obtain an efficient mixing a suitable stirring device with controllable speed is used, as shown in figure 2.

Contaminated seawater, freshwater or brackish water which shall be purified is pumped into the vessel via a pipeline. A purification liquid comprising chitosan mixed with activated carbon, carrageenan and/or bentonit is added to the water in a controlled amount. This addition may be performed in the pipeline feeding water into the vessel, or in the vessel during optionally vigorous stirring of the water in the vessel. The pH in the water may be from pH 7 to pH 3, preferably in the range from pH 4 to pH 6.5.

The anode is then positioned in the vessel as indicated in figure 3. The electrodes may be of any appropriate shape, such as plates, rods, wire nets, etc. They are preferably in the shape of a set of parallel plates, such as anode plates and cathode plates spaced in short distances from each other. Alternate plates will be an anode and a cathode, respectively. A voltage is applied between the anodes and cathodes. The voltage will be in the range from about 0.1 volt to about 500 volts. Because the chitosan particles carry a negative surface charge they will be attracted to the anodes, which have a positive charge, and deposit thereon. The layer on the anodes may grow to a thickness of 2 to 3 cms. and the process will still be equally effective. The contaminations will attach be rather firmly to the anodes. When the process is interrupted or finished the electric current is switched off and the anodes withdrawn from the vessel as indicated in figure 4; and the layer theron is removed and the surfaces of the anodes cleaned. For an efficient operation it is important to use anodes having a large surface.

It is obvious that the present purification method using an electric field may be operated both in batches and in a continuous manner. In a continuous operation the chitosan-containing purification liquid and activated carbon will be added

continuously to the water before the water flows over the suitably shaped electrodes.

Operation parameters such as stirring speed, time period for each stage, flow rates, potential differences between the electrodes, etc, will depend on the equipment used and the level of contaminations in the water. A person skilled i the art will be able to decide on suitable parametres by routine experimentation.

The end product is purified clear water without any objectionable flavor, and free of heavy metals and contaminations. Analysis of the final water shows that the remaining content of contaminations is below the tresholds prescribed by health authorities.

Industrial applicability

The present method provides a simple and cost efficient industrial method for removing contaminations that are precipitated from water by the use of chitosan. The present invention is applicable in all areas of industrial purification of water.

Even if the present invention has been described and exemplified above with a minor selection of marine oils, a person skilled in the art will realize that the present invention can be used for the purification of all types of marine and vegetable oils. Thus, the invention shall not be construed to be restricted to the specific types of oils exemplified, but will encompass all equivalent embodiments. Modifications and variations of the present process will be obvious to a person skilled in the art within the scope of the invention defined by the following claims.