Relations between sorption behavior and electrokinetic remediation effect in soils contaminated with heavy metals

Abstract

Adsorption–desorption is an important mechanism for controlling metal mobility in soil and has a large effect on remediation efficiency. Very little research has examined the relations between heavy metal sorption behaviour and electrokinetic (EK) remediation effect. Four typical soil samples in China (Chernozem, Fluvisol, Yellow brown earth, and Purple soil) were studied. Adsorption isotherms were fit to Freundlich equations. EK removal ratios in the 4 soils were: Purple soil > Yellow brown earth > Fluvisol > Chernozem. Results show that the influencing characteristics significant to Cd EK removal order as follows: total organic carbon (TOC) > 1/n (Freundlich constant) > clay ratio > K (Freundlich constant) > cation exchange capacity > background pH. When 1/n (Freundlich constant) increases by 0.124 or TOC increases by 0.858 g/kg in soils, EK removal ratio (%) of Cd decreases by 1.0%. A ‘2-reactions’ mechanism during EK remediation is speculated, including rapid and slow reactions. Fast reactions are attributed to H+–M2+ exchange processes occurring on the surfaces of different inorganic phases and to the dissolution of the water-insoluble precipitates. Slow irreversible reactions refer to H+ ions exchanging with metal ions bound in the internal lattice sites and organic matter being oxidised by oxidants produced by electrochemical reactions. Slow reactions limit the EK remediation effect.