This invention relates to a method for the removal of heavy metals from soil, mud, sludge and the like by the electro-kinetic process, whereby electric current is used to drive the heavy metals towards electrodes where they can be concentrated in an electrolyte solution from which they can be removed, and an apparatus for the execution of said method and the use hereof.
The need for cleaning soil and areas which are polluted with heavy metals is great and increasing in step with the recognition of the risks which these contaminated areas constitute in both the short and the long view for the environment.
Cleaning, however, is a very complicated and costly process. This means that in the most threatened areas the contaminated earth or sludge must be removed and deposited under controlled conditions for the safety of the surrounding environment.
In other words, the polluted material is isolated and thus the problem of cleaning is postponed until later.
Consequently, there is a great need for a method of cleaning which is suitable for the removal of heavy metals.
Efforts have been made with a so-called electro-kinetic method, whereby electric current is used to drive the heavy metals which exist in the earth in the form of ions towards electrodes where the metal can be concentrated and thereafter removed.
In practice, two electrodes are placed in separate chambers at a suitable distance in the earth. The positive heavy metal ions hereby move towards the negative electrode and the negative heavy metal ions towards the positive electrode.
However, there is a difference in the mobility or the movement of the ions which stem from the electrode process and the heavy metal ions which are bound in the earth. The ions from the electrodes process are the least bound and are herewith of the easiest movability.
There thus arises the problem that a considerable part of the process current will be used to convey ions from the one electrode chamber to the other, so that only a smaller part of the power is utilized in the removal of the heavy metals in the earth.
This means that this known method is not particularly effective and, moreover, uneconomic in relation to the costs involved in the removal and the depositing of earth contaminated with heavy metals, which in turn means that the method has not been taken into practical use.
According to the invention, by using a method whereby the earth is separated from electrodes in chambers, and that there is placed a cation-exchange membrane to prevent anions from the negative electrode chamber from seeping out into the earth and at the same time permit the passage of positive heavy metal ions from the earth and out in the negative electrode chamber, and also placing a second ion exchange membrane to prevent the passage of cations from the positive electrode chamber into the earth and at the same time permit the passage of heavy metal ions from the earth to seep out in the positive electrode chamber, there is achieved the advantage that the ions from the electrode chamber do not pass out into the earth, but remain in the electrolyte solution. The energy is hereby utilized in a far more efficient manner, i.e., solely from the transport of the earth-bound ions to the electrodes, and not for the transport of ions between the electrodes.
According to the invention, it hereby becomes possible to clean products contaminated with heavy metals with a consumption of energy which is particularly competitive with other forms of cleaning, and at the same time to ensure a considerable degree of cleaning effect and herewith the possibility of achieving a degree of cleaning which is surprisingly high.
By using the inventive apparatus comprising an electrode in an electrolyte solution in a chamber separated from a second electrolyte solution in a second chamber by means of ion-exchange membranes, and separating this second electrode chamber from the earth by means of other ion-exchange membranes, an electrode unit which functions satisfactorily is achieved in a surprisingly simple manner.
An optimum effect of the electrode is ensured by isolating the electrolyte solution in the electrode chamber from the remaining electrolyte solution.
By using a circulating electrolyte solution, this can be pumped to a separate electrode chamber where the heavy metals can be deposited and removed from the electrolyte solution.
Finally, it is expedient to use the method and the apparatus for the cleaning of earth, sludge and the like, either alone and/or in connection with a biological cleaning of the earth.