Variations Of Cadmium Activity And Its Controlling Mechanisms In Reductive Soils

Cadmium contamination in soil has been a world-wide problem in decades. Cadmium activity may vary differently in the submerged soils but the mechanisms underlying these variations have been remained conflicting and ambiguous although numerous studies have been conducted to determine the mobility and transformation of Cd in the past. Although some different and reasonable mechanisms have been concluded in the previous literatures in order to explain the increase or decrease of Cd activity in submerged soils, some more restricted visual evidences have not been provided up to now because of the variety of the submerged soil types, the complexity of the characteristics of the submerged soils, the diversity of the external factors and the limitations of the research techniques.In the submerged soils.the iron system is one of the main redox systems. The redox process of iron not only influences the changes in soil surface characteristics, but also brings along the other redox-sensitive elements’redox cycles. In comparison, the change in the valence state as a result of Eh changes and the redox reaction in natural submerged soils are not observed for Cd. Nevertheless, a series of physical, chemical and biological changes as a result of the redox processes in submerged soils will influence directly or indirectly the mobility and transformation of Cd, including adsorption-desorption, precipitation (or co-precipitation)-dissolution, coordination and chelation. Therefore, the redox process of iron under the reductive conditions is likely to be an important factor affecting the activity of Cd. Based on these considerations, two typical soils from the southern China, a red soil and a fluvo-aquic soil, were sampled to investigate the mobility and transformation of Cd under the reductive conditions.Firstly, the relationship between the different fractions of Cd among soil solid phases and the CAI (Cadmium-availability index) after flooding was examined through the static incubation experiment. Secondly, the continuous N2bubbling experiment was conducted to study the variations of Cd activity and investigate the mechanisms underlying these variations within the different ranges of pH and pe+pH values, as well as the minerals controlling the solubility of Fe (Ⅱ). Finally. the possible mechanisms controlling the transformation of Cd among several different soil solid phases were elucidated on the basis of the relationship between the Fe distribution and the Cd distribution. On one hand, the mechanisms controlling the solubility of Cd were investigated and elucidated according to the different ranges of pH and pe+pH values. On the other hand, the mechanisms controlling the transformation among the soil solid phases of Cd were speculated through the iron-based redox processes for the first time. Therefore, the results of this study not only extend the basic theory of soil chemistry, but also are of great significance for the remediation of the Cd contaminated soil. The major experimental results were summarized below:1) Compared to nonflooding. flooding of the red and fluvo-aquic soils not only decreased the CAI, but also decreased the exchangeable Cd concentration and increased the carbonate-and Fe-Mn oxides-bound Cd concentrations significantly. Stepwise linear regression analysis for the CAI and the different fractions of Cd among solid phases of the flooded red and fluvo-aquic soils showed that the exchangeable Cd concentration correlated positively with the CAI and did negatively with the Fe-Mn oxides-bound Cd concentration, respectively, indicating that the Cd transformation from the exchangeable fraction to the Fe-Mn oxides-bound fraction was the main reason for the CAI decrease after flooding of the red and fluvo-aquic soils.2) The Fe (Ⅱ) solubility increased during the continuous N2bubbling experiment of the red or fluvo-aquic soil. For the red soil, the reductive dissolution of Fe3O4(magnetite) took place when pH increased from5.51to5.65and pe+pH decreased from9.58to8.60, whereas the reductive dissolution of γ-Fe2O3(maghemite) did when pH increased from5.70to5.82and pe+pH decreased from8.28to7.52. However, for the fluvo-aquic soil, the Fe3(OH)8(ferrosic hydroxide) begun to dissolve reductively when pH fluctuated between7.52and7.56and pe+pH did between8.93-8.47.3) As the degree of reduction increased, the solubility of Cd in the red soil increased continuously, while in the fluvo-aquic soil increased first and then decreased. More importantly, whether the red soil or the fluvo-aquic soil, the mechanisms controlling the Cd solubility were different within the different ranges of pH and pe+pH values. For the red soil, the decrease of pH. the reductive dissolution of Mn minerals and the competitive adsorption of DOC,Al3+, K+and Mg2+for the soil surface adsorptive positions led to the increase of the Cd solubility when pH decreased from5.61to5.51and pe+pH did from11.09to9.67, whereas the decomposition of DOC. the reductive dissolutions of Fe3O4(magnetite),γ-Fe2O3(maghemite) and Mn minerals and the competitive adsorptions of Mg2+and K+for the soil surface adsorptive positions accelerated the release of Cd to the soil solution when pH increased from5.51to5.82and pe+pH decreased from8.91to7.52. In comparison, the competitive adsorption of Ca2+, K+, Mg2+and Na+for the soil surface adsorptive positions and the chelation of DOC with Cd2+increased the solubility of Cd when pe+pH decreased from11.29to9.10, whereas the solubility of Cd decreased due to the decomposition of DOC and the adsorption of Cd2+onto the surfaces of neoformed Fe and carbonates minerals when pe+pH fluctuated between8.93-8.47. The results of stepwise linear regression analysis showed the decrease of pH and the reductive dissolutions of Fe3O4(magnetite) and γ-Fe2O3(maghemite) were the major mechanisms leading to the increase of Cd solubility in the red soil, whereas the increase and decrease of Cd solubility in the fluvo-aquic soil were primarily due to the release and decomposition of DOC.4) Whether in the red soil or the fluvo-aquic soil, the Fe redistribution and the Cd redistribution among the soil solid phases took place when soil became more reducing. And. the mechanisms controlling the transformation of Cd among several chemical fractions were speculated on the basis of the relationship between the Fe redistribution and the Cd redistribution. For the red soil, the reductive dissolutions of Fe3O4(magnetite) and γ-Fe2O3(maghemite) and their impacts on the soil surface chemical properties resulted in the transformation of Cd from the Fe-Mn oxides-bound fraction to the carbonate-bound fraction. However, for the fluvo-aquic soil, the readsorptions of Cd onto the carbonates surface covered by Fe resulting from the reductive dissolution of Fe3(OH)8(ferrosic hydroxide) and onto the neoformed surfaces of Fe minerals induced the transformation of Cd from the exchangeable fraction to the carbonate-and Fe-Mn oxides-bound fractions.