WO2004033374A1

WO2004033374A1 - Method and medium for the removal of heavy metals from aqueous solutions

Info

Publication number: WO2004033374A1
Application number: PCT/FI2003/000738
Authority: WO
Inventors: Vesa Kettunen
Original assignee: Kemira Oyj
Priority date: 2002-10-08
Filing date: 2003-10-07
Publication date: 2004-04-22
Also published as: FI20021787A0; AU2003268974A1; FI20021787A; FI117702B

Abstract

The invention relates to a method for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same, comprising contacting an aqueous solution comprising contaminant heavy metals and optionally phosphorus with an active medium comprising a drinking water sludge containing iron ions and a pH adjustment agent, said active medium being capable of binding the contaminant heavy metals and phosphorus, if present, thereby producing a purified solution. The invention also relates to such an active medium.

Description

METHOD AND MEDIUM FOR THE REMOVAL OF HEAVY METALS FROM AQUEOUS SOLUTIONS

The present invention relates to a method for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same. Said aqueous solution is especially an aqueous solution brought by means of rain water or melt water from a contaminated soil area. The invention also relates to an active medium for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same.

In this specification the term "heavy metals" includes at least one of following elements:

Pb, Cr, As, Cu, Mo, Cd, Ni and Zn

Heavy metals are highly toxic and constitute a serious risk to people and the environment. Heavy metals in the soil cause contamination in several places in the world. Typical places contaminated by heavy metals are old industrial areas, dumping areas and shooting ranges. The heavy metal content in the soils of such areas can be very high. The heavy metals are usually slightly soluble in water and so, for example, the rainwaters may dissolve them. The dissolved heavy metals cause the contamination of ground waters or surface waters depending on where the dissolved heavy metals will drift.

The current techniques applied for solving these problems are rather expensive and are currently applied only in rather limited application areas. In many cases the heavy metals that are dissolved from the contaminated soil are not treated at all. Due to this there is in this field a need for effective and cheap treatment methods.

Phosphorus exists in both solid and liquid form in wastewaters. There are standard processes for phosphorus removal from for example municipal wastewaters by means of traditional coagulation techniques.

It is known that heavy metals may be removed by filtering the wastewater through a fixed bed containing ferric oxide and/or ferric hydroxide. This kind of processes have been developed especially for arsenic removal applications. In the patent JP 2001070954 "Agents for stripping heavy metals from groundwater" a fixed bed of iron ore or sand containing mainly Fe oxide and Fe hydroxide is used for the heavy metals removal. It is stated that Cd, Cr, Pb and As can be removed with this kind of process. One problem of this technique is the price of synthetic Fe oxide and hydroxide. The high price of these synthetic materials increases the purification costs and limits the application area.

Some researchers have investigated the use of Fe-bearing materials in the heavy metals immobilization. One such material is drinking water sludge. The results are presented in the articles: "Immobilization of heavy metals in sediment dredged from a seaport by iron bearing materials" (Wat. Sci. Tech. Vol. 37, No. 6-7, pp. 379-386, 1998) and "Use of Fe-containing residues as soil additive for monitoring heavy metal-polluted locations in Harzvorland" (Bergbau und Umwelt: Langfristige Geochemische Einfluesse, 2000, 199-207). The hazardous substances investigated in these studies were Cd, Zn, Pb and Cu. The aim was to reduce the uptake of heavy metals by plants grown on polluted soil. The problem of this method is the rather poor immobilization efficiency. Typically the uptake of heavy metals by plants was reduced by 20-50%. For example for Pb an immobilization efficiency of 40-60% was achieved. These articles do not deal with the removal of heavy metals from contaminated soil. In heavy metal removal processes significantly better efficiency is usually needed.

One method for the heavy metal removal from contaminated soil is based on mechanically removing all the contaminated soil and transferring the same to a specially designed piling area. This method is very expensive but still widely used in many countries.

According to the present invention there is provided a method for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same, comprising contacting an aqueous solution comprising contaminant heavy metals and optionally phosphorus with an active medium comprising a drinking water sludge containing iron ions and a pH adjustment agent, said active medium being capable of binding the contaminant heavy metals and phosphorus, if present, thereby producing a purified solution.

When the aqueous solution comprising contaminant heavy metals and optionally phosphorus is in contact with the active medium of the invention, chemical reactions are taking place and the contaminant substances are adsorbed and/or precipitated to the active medium and therefore the solution is purified from contaminants. The present invention also provides an active medium for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same, comprising a drinking water sludge containing iron ions and a pH adjustment agent, and optionally a permeability improvement substance.

A further aspect of the present invention is the use of an active medium comprising a drinking water sludge containing iron ions and a pH adjustment agent, and optionally a permeability improvement substance for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same.

By using the active medium according to the present invention the raw material costs in the purification process can be significantly decreased. This can be achieved because synthetic raw materials are not needed. At the same time the sludge disposal costs are decreased in the drinking water treatment plants. Some drinking water treatment plants release the sludge to the sea. In such cases the sludge disposal may cause esthetic and/or odor problems that can be avoided by reutilizing the sludge.

The major benefit of the active medium of the invention is the high efficiency. A removal efficiency of at least 98% was achieved for all tested heavy metals and for phosphorus. The permeability of the active medium of the invention can also be adjusted so that it will be suitable for each specific application.

The composition and structure of the active medium of the present invention

The active medium of the present invention comprises a drinking water sludge containing iron ions and a pH adjustment agent and optionally a permeability improvement substance.

The need of the third component, i.e. the permeability improvement substance, depends on how the first and second components are treated and what is the specific application for the active medium. In the purification process the components of the active medium may be mixed together or placed in the system in specially designed layers. Further, a part of active medium or all the active medium may be granulated or extruded or treated in some other way before use in order to increase the permeability.

The active medium of the present invention can be used for example in fixed bed filters. It can also be used in other purification processes where the contaminated solution is purified by passing the same through the active medium or by reacting the same with the active medium and then separate the active medium from the purified solution.

1. Drinking water sludge

The first component of the active medium is the drinking water sludge containing iron ions, preferably Fe³⁺. The sludge may be formed in any kind of drinking water purification processes where coagulants containing Fe³⁺ and/or Fe²⁺ are used. Practically the only limitation is that it should not contain high concentrations of such substances that are hazardous to the environment. The iron containing coagulants that are most often used in drinking water purification processes are ferric sulphates, ferric chlorides or mixtures thereof. The drinking water sludge is dewatered to some extent before it can be use in the active medium preparation. If the sludge contains Fe²⁺ it is preferable to oxidize the same to Fe³⁺. The oxidant may be for example 0₂, H₂0₂, Cl₂, NaCIO, NaC10₃ or theoretically any other

94- » ^4- substance that can oxidize Fe in the sludge to Fe .

The dewatering degree may vary depending on the planned application for the active medium. The dry content of the sludge after dewatering may vary between 1- 100% by weight depending on the application. The dry content is preferably 10- 50%) by weight and more preferably 30-50% by weight.

The Fe content of the sludge may also vary depending on the dry content and impurities. The Fe content of the sludge may be 3-50%, preferably 20-40% by weight of the dry content of the sludge. Correspondingly the Fe content of the sludge may be 0.2-45%, preferably 3-20% and more preferably 10-20% by weight of the sludge.

The amount of the drinking water sludge in the active medium strongly depends on the process design. It may be 1-99.95%) (w/w) of the total active medium weight. Usually the preferred amount is 5-60% (w/w) and more preferably 10-50% (w/w) of the active media.

2. pH adjustment agent

The second component of the active medium is the pH adjustment agent. This agent may be any substance that is not harmful to the environment and is able to increase the pH of the active medium to the targeted level. The pH adjustment agent should also have a high buffering capacity so that the pH of the active medium can be unchanged for long times during the purification process.

The pH adjustment agent can be selected from the group consisting of hydroxides, oxides and carbonates of alkali metals and alkaline-earth metals and mixtures thereof. The pH adjustment agent is preferably CaC0₃, CaO or Ca(OH)₂. Other suitable caustic substances that can be used for this purpose are for example MgO, Mg(OH)₂, Na₂C0₃ and NaOH. Also mixtures of these substances can be used as well as mixed compounds such as dolomite (MgO-CaC0₃).

The targeted pH level of the active medium depends on the element that should be removed from the contaminated solution. The pH of the active medium may vary between 4-13 in the removal of heavy metals and phosphorus. Usually the more preferred pH is 6-12. The most preferred pH for Pb, Cr, Cu, As, Mo and P removal is 7-10.

The amount of the pH adjustment agent may be 0.05-50% (w/w) of the weight of the drinking water sludge. Preferably the weight is 0.05-20%) (w/w) and more preferably 0.05-10% (w/w) of the weight of the drinking water sludge.

3. Permeability improvement substance

The third component of the active medium is the permeability improvement substance. This component may be any material that is not harmful to the environment and that is coarse enough in order to increase the permeability of the active medium. The preferred materials are sand, rubble, crude ore, country rock, ore and some granulated waste materials like granulated fly ash.

It is also possible to granulate or extrude the drinking water sludge and the pH adjustment agent without using the permeability improvement substance, or treat the sludge some other way in order to increase its permeability. It is possible that after this kind of treatment the permeability improvement substance is not needed in the active medium.

The amount of the permeability improvement substance in the active medium depends on the quality of the drinking water sludge to be used, the targeted permeability in the process as well as on the process design. The amount of the permeability improvement substance may be 0-95% (w/w) of the total weight of the active medium. The needed amount strongly depends on the process design. Usually the preferred amount is 10-80% (w/w) and more preferable 50-75% (w/w) of the weight of active medium.

The active medium of the present invention can be prepared by first mixing the drinking water sludge and the pH adjustment agent, and then adding the permeability improvement substance to obtain a homogeneous mixture. This mixture can then be extruded or alternatively granulated. If the permeability improvement substance is omitted, the active medium of the present invention can be prepared by mixing the drinking water sludge and the pH adjustment agent. This mixture can then be extruded or alternatively granulated.

There are different modes of using the active medium of the present invention in the removal of heavy metals or heavy metals and phosphorus.

In a preferred embodiment of the invention applied to the removal of heavy metals from a contaminated soil area, such as an old industrial area, dumping area or shooting range, the route of leaching water in the contaminated soil area is first surveyed, and then based on the obtained data the active medium is placed in the route of leaching water in such a way that leaching water from the contaminated soil is passed through the active medium. Such a system can be called soil filter or reactive wall. The heavy metals are stopped by the active medium and the water ending up in the groundwater, lakes, sea or rivers is free or essentially free from heavy metals. The required amount of active medium is evaluated for example by analyzing soil samples. Additionally, the required amount of active medium depends on the target operating time of the system. The operating time can for example be between 10 and 50 years. The quality of the filtrate water is regularly controlled. If the quality is getting worse the active medium is changed for a new one.

In another embodiment of the invention contaminated leaching water or wastewater containing heavy metals and optionally phosphorus from a contaminated soil area is conducted into a storage container. From this container the contaminated water is conducted into another container wherein the contaminated water is contacted with the active medium. If filtering is causing problems the permeability improvement substance can be present. In this embodiment of the invention the drinking water sludge and the pH adjustment agent can be added to the water to be treated as a mixture or separately. Thereafter the mixture is mixed for a suitable period of time. Thereafter the mixture is filtered and the filtrate free or essentially free from heavy metals and phosphorus can be fed to a lake, sea or river. The obtained precipitate containing heavy metals can be reused but at some stage it has to be withdrawn from the purification system and replaced by fresh active medium.

In this specification the percentages are % by weight unless otherwise specified.

The present invention will be described in more detail in following examples, wherein Examples 1 to 3 describe the preparation of the active medium of the present invention, and Examples 4 to 6 describe some experiments carried out in order to evaluate the efficiency of the active medium of the present invention in the removal of heavy metals and phosphorus. These experiments were carried out as 24 h shaking tests followed by vacuum filtration. In the shaking test the automatic Heidolph PROMAX 1020 shaker was used. The shaking speed was 200 φm. Two different contaminant solutions with following composition were tested:

Contaminant solution 1 Contaminant solution 2

Pb 52 ppm P 57 ppm

Cu 44ppm Mo 350 ppm

Cr 52 ppm

As 53 ppm

Example 1

100 g of 1% sludge is taken from a drinking water purification plant. The dry content of the sludge is increased to 8% using a centrifuge. The Fe concentration of this sludge is 2.5%. The weight of 8% sludge for the active medium preparation is 12.5 g. The pH of the sludge is adjusted to 8 with 0.08 g of CaO. 35.0 g of sand is then added to the active medium so that a homogeneous mixture is achieved. The homogeneous mixture is then extruded. The prepared active medium has following composition:

Drinking water sludge 26.3% pH adjustment agent 0.17%

Permeability improvement substance 73.6%>

Example 2

1000 g of 1%) sludge is taken from a drinking water purification plant. The dry content of the sludge is increased to 30% using a centrifuge. The Fe concentration of this sludge is 9.5%>. The weight of 30% sludge for the active medium preparation is 33.3 g. The pH of the sludge is adjusted to 10 with 1.4 g of CaO. 35.0 g of sand is then added to the active medium so that a homogeneous mixture is achieved. The mixture is then granulated. The prepared active medium has following composition:

Drinking water sludge 47.8% pH adjustment agent 2.0% Permeability improvement substance 50.2%

Example 3

lOOOg of 1% sludge is taken from a drinking water purification plant. The dry content of the sludge is increased to 50% using a filter press. The Fe concentration of this sludge is 15.8%. The weight of 50% sludge for the active medium preparation is 20.0 g. The pH of the sludge is adjusted to 8 with 0.8 g of CaO. The mixture is then granulated. The prepared active medium has following composition:

Drinking water sludge 96.2% pH adjustment agent 3.8%>

Permeability improvement substance 0%

Example 4

12.5 g of 8% drinking water sludge (Fe concentration 2.5%) was mixed with 100 g of Contaminant solution 1. The pH was adjusted to 8 with 0.08 g CaO. The composition of the active medium was as follows:

Drinking water sludge 99.4% pH adjustment agent 0.6%

The mixture was shaken 24 h with the automatic shaker at 200 φm. After this the mixture was filtered with vacuum filter and the contaminant levels in the filtrate were analyzed. The results are presented in Table 1 below.

Table 1

Example 5

125.0 g of 8%o drinking water sludge (Fe concentration 2.5%) was mixed with 100 g of Contaminant solution 2. The pH was adjusted to 8 with 0.8 g CaO. The composition of the active medium was as follows:

Drinking water sludge 99.4% pH adjustment agent 0.6%

The mixture was shaken 24 h with the automatic shaker at 200 φm. After this the mixture was filtered with vacuum filter and the contaminant levels in the filtrate were analyzed. The results are presented in Table 2 below.

Table 2

Example 6

25.0 g of 8%_> drinking water sludge (Fe concentration 2.5%) was mixed with 100 g of Contaminant solution 1. The pH was adjusted to 10 with 0.3 g CaO. The composition of the active medium was as follows:

Drinking water sludge 98.8% pH adjustment agent 1.2%

The mixture was shaken 24 h with the automatic shaker at 200 φm. After this the mixture was filtered with vacuum filter and the contaminant levels in the filtrate were analyzed. The results are presented in Table 3 below. Table 3

Claims

1. A method for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same, comprising contacting an aqueous solution comprising contaminant heavy metals and optionally phosphorus with an active medium comprising a drinking water sludge containing iron ions and a pH adjustment agent, said active medium being capable of binding the contaminant heavy metals and phosphorus, if present, thereby producing a purified solution.

2. The method according to claim 1, wherein the drinking water sludge comprises ferric ions.

3. The method according to claim 1 or 2, wherein the dry content of the drinking water sludge is between 1% and 100% by weight, preferably between 10% and 50% by weight.

4. The method according to any of claims 1 to 3, wherein the iron content of the drinking water sludge is between 3% and 50%, preferably between 20% and 40% by weight of the dry content of the sludge.

5. The method according to any of claims 1 to 4, wherein the pH adjustment agent is selected from the group consisting of hydroxides, oxides and carbonates of alkali metals and alkaline-earth metals and mixtures thereof, such as calcium carbonate, oxide or hydroxide, magnesium oxide or hydroxide, or sodium carbonate or hydroxide or mixtures thereof:

6. The method according to any of claims 1 to 5, wherein the amount of the pH adjustment agent is from 0.05%> to 50% of the weight of the drinking water sludge.

7. The method according to any of claims 1 to 6, wherein the active medium additionally comprises a permeability improvement substance.

8. The method according to claim 7, wherein the permeability improvement substance is selected from the group consisting of sand, rubble, crude ore, country rock, ore and granulated waste materials such as granulated fly ash.

9. The method according to any of claims 1 to 8, wherein the active medium is in the form of a granulated or extruded product.

10. The method according to any of claims 1 to 9, wherein said aqueous solution comprises an aqueous solution containing heavy metals brought by means of rain water or melt water from a contaminated soil area.

11. The method according to claim 10, wherein the active medium is placed in the soil area in such a way that the aqueous solution is passed through the active medium.

12. The method according to claim 10, wherein the aqueous solution is conducted into a container wherein the solution is contacted with the active medium.

13. The method according to claim 12, wherein the drinking water sludge and the pH adjustment agent are added separately into the container.

14. An active medium for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same, comprising a drinking water sludge containing iron ions and a pH adjustment agent, and optionally a permeability improvement substance.

15. The medium according to claim 14 wherein all the components of the active medium are in the form of a mixture, such as a granulated or extruded mixture.

16. Use of an active medium comprising a drinking water sludge containing iron ions and a pH adjustment agent, and optionally a permeability improvement substance for the removal of contaminant heavy metals and optionally phosphorus from an aqueous solution containing the same.