|1.||A method of removing heavy metals from soil, sludge, compost and similar materials, c h a r a c t e r i z e d in that the heavy metal containing material is treated with an agent forming a complex with heavy metal ions so as to form a product consisting of a solid phase and a liquid phase containing a heavy metal complex, in that the liquid phase is separated off, in that the liquid phase separa¬ ted off is treated with an ion exchanger so as to bond heavy metals onto the ion exchanger, and in that the heavy metals are optionally removed from the ion exchanger by eluation.|
|2.||A method according to claim 1, c h a r a c t e r i z e d in that the heavy metal containing material is aired prior to the treatment with the complex bonding agent.|
|3.||A method according to claim 1 or 2, c h a r a c t e r i z e d in that EDTA is used as complex bonding agent.|
|4.||A method according to claim 3, c h a r a c t e r i z e d in that EDTA is used as complex bonding agent in a concentration ranging from about 5 to about 50 mmol per litre of heavy metal containing material.|
|5.||A method according to claim 3 or 4, c h a r a c t e r i z e d in that additionally pyrophosphate is used as complex bonding agent.|
|6.||A method according to claim 5, c h a r a c t e r i z e d in that pyrophosphate is used as complex bonding agent in a concentration ranging from about 1 to about 50 mmol per litre of heavy metal containing material.|
|7.||A method according to any of the preceding claims, c h a r a c ¬ t e r i z e d in that a bark ion exchanger is used as ion exchan ger.|
|8.||A method according to claim 7, c h a r a c t e r i z e d in that a sulphonated bark ion exchanger is used as bark ion exchanger.|
|9.||A method according to any of the preceding claims, c h a r a c ¬ t e r i z e d in that the pH value of the mixture of the heavy metal containing material and the complex bonding agent is adjusted from about 3 to about 5.5.|
|10.||A method according to any of the preceding claims, c h a r ¬ a c t e r i z e d in that the liquid separated off is treated on the ion exchanger for a period of from about 30 to about 120 mi¬ nutes.|
The present invention relates to a method of removing heavy metals from soil, sludge, compost and similar materials.
In recent years the deposition and use of sludge and compost have become an increasing problem due to the content of heavy metals. Thus, it is impossible to use sludge from e.g. municipal purifica¬ tion plants and having a high content of heavy metals for compost¬ ing. Furthermore, in many cases it is not possible to deposit heavy metal containing sludge directly at the most natural site, viz. agricultural land. Consequently, it becomes necessary to deposit the sludge at disposal sites or burn it, which alternatives both cause other environmental problems and entail considerable costs.
Furthermore, many building sites and land areas, such as old chemi¬ cal depots, are contaminated with heavy metals.
A known method of removing heavy metals from soil, sludge or compost comprises extracting the heavy metals with acid. However, the method suffers from the drawback that the heavy metal containing extract constitutes a problem of almost the same magnitude as the heavy metal containing material itself as the extract must either be disposed of by depositing, which is expensive and causes environ¬ mental problems or by processing at a special chemical reception plant which would in most cases also be too expensive. A further drawback of the known method is that in extraction of sludge a very low pH value is often required as the heavy metals are present in a strongly bonded state in the dead cells in the sludge. Consequently, large amounts of acid and base are required to lower and readjust, respectively, the pH value.
Thus, there is a considerable need for the development of new methods of removing heavy metals from soil, sludge, compost and similar materials.
The object of the present invention is to provide a method of
removing heavy metals from soil, sludge, compost and similar mate¬ rials, whereby the heavy metals are concentrated to a larger extent than is the case in connection with the known methods.
This object is obtained according to the invention by a method which is characterized in that the heavy metal containing material is treated with an agent forming a complex with heavy metal ions so as to form a product consisting of a solid phase and a liquid phase containing a heavy metal complex, in that the liquid phase is separated off, in that the liquid phase separated off is treated with an ion exchanger so as to bond heavy metals onto the ion exchanger, and in that the heavy metals are optionally removed from the ion exchanger by eluation.
It has surprisingly been found that by using the method of the invention both an efficient removal of heavy metals from the star¬ ting material and a very substantial concentration of the heavy metals removed can be obtained, whereby the final disposal of the extracted heavy metals is considerably facilitated.
By practicing the method of the invention it has e.g. been possible to remove up to 60% of the heavy metals and to obtain an end product having a concentration of heavy metals which is 800 times that of the starting material.
In a preferred embodiment of the method of the present invention, whereby the heavy metal containing material is aqueous, the heavy metal containing material is treated with a solid complex bonding agent in order to avoid an increase of the volume of the treated material. If the starting material is too dry the complex bonding agent is added as an aqueous solution. The pH value of the mixture of the heavy metal containing material and the complex bonding agent should preferably be in the range of about 3 to 5.5 and if necessary an adjustment of the pH value is carried out by using e.g. nitrous acid or sulphuric acid. However, the starting material will often have a pH value of about 5. When treating sludge which is subjected to a dewatering process using e.g. a sieve diaphragm press/plate and frame filter or a centrifuge the complex bonding agent may advan¬ tageously be added in solid form before the dewatering.
The treatment with the complex bonding agent may optionally be repeated one or more times in order to obtain a desired degree of removal of heavy metals.
In connection with the present invention EDTA or a mixture of EDTA and pyrophosphate is preferably used as complex bonding agent. When using EDTA as a complex bonding agent it is preferably used in a concentration of from about 5 to about 50 mmol per litre of heavy metal containing material. When using pyrophosphate it is preferably used in a concentration in the range of about 1 to about 50 mmol per litre of heavy metal containing material.
Subsequently, the solid matter is separated from the liquid, which separation may be carried out by e.g. sedimentation, centrifugation or filtration.
The separated liquid is then treated on an ion exchanger. Preferab¬ ly, an ion exchanger based on bark is used, such as a sulphonated bark ion exchanger, as such ion exchangers are inexpensive. A sulphonated bark ion exchanger is capable of specifically absorbing about 0.3 equivalent heavy metal per litre. By "specifically" is meant that the absorption can take place without the influence of other ions possibly present, such as sodium ions and calcium ions. The overall capacity of the bark ion exchanger corresponds to about 1 equivalent per litre, but only about 30% of the ionic groups of the ion exchanger are specific to the absorption of heavy metals. The duration of the treatment on the ion exchanger is preferably in the range of about 30 to about 120 minutes.
Finally, the collected heavy metals may optionally be eluted from the ion exchanger by e.g. acid so as to obtain the heavy metals. Alternatively, the bark ion exchanger may be burnt using an exhaust gas filter whereby the heavy metals are further concentrated in the ashes.
Prior to the treatment with the complex bonding agent the heavy metal containing material may be aired, whereby a considerably improved extraction of the heavy metals from the heavy metal con¬ taining material is obtained. The further treatment of the extract
on an inexpensive bark ion exchanger in addition to the above discussed substantial concentration provides the advantage that the problem of the expensive treatment or depositing of the environmen¬ tally harmful extract is avoided and that the liquid used for the extractions may be regenerated and recycled, whereby the expenses for chemicals for the extraction are reduced considerably.
The invention will now be described in further detail with reference to the following examples.
1 kg matured compost (wet basis) was aired for 30 minutes, and 20 mmol EDTA, 5 mmol pyrophosphate and 100 ml water were then added to reduce the content of dry matter to 60 % by weight. The compost contained 105 ppm lead and 4 ppm cadmium (wet basis), which it was desired to reduce to 40 ppm lead and 1 ppm cadmium (dry matter basis). After 15 minutes a pH value of 5.0 was measured. Then the compost was sedimented, and the supernatant was removed and centri- fuged thereby obtaining 110 ml clear liquid which contained 77 mg lead and 2.4 mg cadmium corresponding to a reduction of the content of the compost to 28 ppm lead and 1.6 ppm cadmium on a wet basis. To obtain the desired reduction it was thus necessary to perform a further extraction resulting in an additional 100 ml of clear liquid containing 17 ppm lead and 1.0 ppm cadmium, and the content of lead and cadmium of the compost had then been reduced to the desired level.
The 210 ml of extract were then treated on a sulphonated bark ion exchange column having a total volume of 100 ml. The period of treatment was about 2 hours the calculated residence time in the column being about 1 hour. The concentration of the extract of 52 ppm lead and 1.7 ppm cadmium was reduced to 1 ppm lead and 0.02 ppm cadmium by the treatment.
In order to determine the compost treatment capacity of the bark ion exchanger a total of 12 kg of compost were treated on the ion exchange column without it showing any sign of saturation. The ion exchanger was then saturated using water containing lead and cadmium
in the same concentration as the compost extract (52 ppm lead and 1.7 ppm cadmium), and it was found that 100 ml of bark ion exchanger is capable of retaining heavy metals from about 20 kg of compost, which corresponds to a concentration of heavy metals from the compost to the bark ion exchanger of about 800 times.
1.2 kg of harbour sludge having a content of dry matter of 32% was aired for 2.5 hours. 50 mmol EDTA and 1.5 ml concentrated sulphuric acid were then added, and the sludge was then centrifuged. The clear supernatant had a volume of 350 ml and a pH value of 4.3 and con¬ tained 1840 ppm lead, 23 ppm cadmium, 560 ppm chromium and 1230 ppm copper. Before the extraction the harbour sludge contained 2100 ppm lead, 41 ppm cadmium, 760 ppm chromium and 1450 ppm copper on a dry matter basis, and after the extraction it contained 235 ppm lead, 17 ppm cadmium, 184 ppm chromium and 210 ppm copper.
The extract was treated on a 200 ml bark ion exchange column having a residence time of 45 minutes, whereby the heavy metal content of the extract was reduced to 12 ppm lead, 2 ppm cadmium, 12 ppm chromium and 2 ppm copper. The bark ion exchanger was saturated by treatment with a total of 8 times 1.2 kg harbour sludge correspond¬ ing to a concentration of 48 times.
200 g of soil was treated with 50 ml 30 mM EDTA plus 10 ml 25 mM pyrophosphate. Following 10 minutes of aeration and subsequent sedimentation the clear aqueous phase of 55 ml in all was separated off. In the extraction the lead content of the soil was reduced from 97 ppm to 31 ppm (dry matter basis).
The extract, which contained 208 ppm lead, was treated on a 100 ml bark ion exchange column having a residence time of 1 hour. By use of a liquid having the same heavy metal concentration as the extract it was determined that a total of 60 soil samples of 200 g each can be treated on the ion exchange column before it shows any sign of saturation. This corresponds to a concentration of 120 times.