Soil cleaning process

Abstract

A process for cleaning contaminated soil containing a heavy metal, comprising: (a) an extraction step where extracting the heavy metal contained in the soil as a heavy metal ion by bringing an extracting reagent into contact with the soil to obtain an extract “a” containing soil and the heavy metal ion; (b) a solid-liquid separation step where carrying out the solid-liquid separation of the extract “a” to obtain cleaned soil and an extract “b” containing the heavy metal ion; (c) a precipitation step where adjusting the pH of the extract “b” to 3 or more in the presence of an iron ion to precipitate the heavy metal together with ion so as to obtain an extract “c” containing iron and heavy metal precipitates; and (d) a heavy metal recovery step where foaming the extract “c” in the presence of a surfactant and collecting the iron and heavy metal precipitates by bubbles to recover them. The process can remove a heavy metal from the polluted soil containing the heavy metal efficiently.

1 . A process for cleaning contaminated soil containing a heavy metal, comprising: 
 (a) an extraction step where extracting the heavy metal contained in the soil as a heavy metal ion by bringing an extracting reagent into contact with the soil to obtain an extract “a” containing soil and the heavy metal ion;    (b) a solid-liquid separation step where carrying out the solid-liquid separation of the extract “a” to obtain cleaned soil and an extract “b” containing the heavy metal ion;    (c) a precipitation step where adjusting the pH of the extract “b” to 3 or more in the presence of an iron ion to precipitate the heavy metal together with ion so as to obtain an extract “c” containing iron and heavy metal precipitates; and    (d) a heavy metal recovery step where foaming the extract “c” in the presence of a surfactant and collecting the iron and heavy metal precipitates by bubbles to recover them.    
     
     
         2 . The process for cleaning soil according to  claim 1 , wherein a dissolved heavy metal in the extract “c” is collected by the formed bubbles in the heavy metal recovery step to recover it.  
     
     
         3 . The process for cleaning soil according to  claim 2 , wherein the dissolved heavy metal is a metal complex.  
     
     
         4 . The process for cleaning soil according to  claim 1 , wherein the contaminated soil contains at least one heavy metal selected from the group consisting of cadmium, copper, antimony, lead, arsenic, chromium, mercury and zinc.  
     
     
         5 . The process for cleaning soil according to  claim 4 , wherein the contaminated soil contains iron.  
     
     
         6 . The process for cleaning soil according to  claim 1 , wherein the extracting reagent is an aqueous solution of hydrochloric acid.  
     
     
         7 . The process for cleaning soil according to  claim 1 , wherein at least one selected from the group consisting of iron chloride, iron nitrate, iron sulfate and iron acetate is added to the extract “b” in the precipitation step to have an iron ion existent in the extract “b”, and then the pH of the extract “b” is adjusted to 3 or more.  
     
     
         8 . The process for cleaning soil according to  claim 1 , wherein an alkali agent is added to the extract “b” to adjust its pH to 3 or more in the precipitation step.  
     
     
         9 . The process for cleaning soil according to  claim 8 , wherein the alkali agent is sodium hydroxide and/or potassium hydroxide.  
     
     
         10 . The process for cleaning soil according to  claim 1 , wherein the surfactant is an anionic surfactant and/or a cationic surfactant.  
     
     
         11 . The process for cleaning soil according to  claim 1 , wherein a foaming agent is used in combination with the surfactant in the heavy metal recovery step.  
     
     
         12 . The process for cleaning soil according to  claim 1 , wherein the solid-liquid separation step comprises the following substeps: 
 (b-1) carrying out the solid-liquid separation of the extract “a” so as to obtain cleaned soil and an extract “b- 1 ” containing the heavy metal ion and fine particles of the soil;    (b-2) coagulating the extract “b- 1 ” to obtain an extract “b- 2 ” containing agglomerated particles and the heavy metal ion; and    (b-3) carrying out the solid-liquid separation of the extract “b- 2 ” so as to obtain solid matter comprising agglomerated particles and the extract “b” containing the heavy metal ion.    
     
     
         13 . The process for cleaning soil according to  claim 12 , wherein the particle diameter of fine particles of the soil contained in the extract “b- 1 ” is 100 μm or less.  
     
     
         14 . The process for cleaning soil according to  claim 12 , wherein the solid matter separated in the substep (b-3) is returned into the extract “a” in the substep (b-1).