Effect Of Humic Fracions On The Retention Process And Occurrence Forms Of Chromium In Black Soil

At present,chromium has become one of the most common heavy metal contaminants in soils of China.The soil organic matter has a great retention effect on Cr?VI?migration,and plays an important role in the transformation of Cr?III?and Cr?VI?.In this paper,the adsorption experiment of Cr?VI?on black soil is carried out with Dunhua black soil as representative.FTIR spectroscopic technique,UV-Vis spectral analysis and improved Tessier speciation metohd were used to explore the mechanisms of retention process and occurrence forms of Cr?VI?between different humus fractions.The main conclusions and understanding are listed as following:?1?The Tessier sequential extraction method was improved based on humus classification,and the traditional organic matter bound state?OM?was divided into extractable humus bound state?HE?and unextractable humus bound state?HU?.The improved method was tested using the spiked recovery method and the contaminated soil sample calibration method.The recoveries of total chromium in both soil and its respective occurrence forms were within the acceptable error range of±10%,which demonstrating that the accuracy and precision of the improved method met the requirements.?2?The retention process of Cr?VI?in humus-rich Dunhua black soil obeyed the"adsorption-reduction"mechanism.80.31%¹00%of the Cr?VI?adsorbed is reduced to Cr?III?,and only a small part of Cr?VI?exists with the adsorption state.The content of adsorbed Cr?VI?was determined by the solid-liquid partition coefficient K_d..Soil humus is the main electron donor for Cr?VI?reduction in test soil,and the reduction rate of Cr?VI?is determined by the concentration of H⁺,the adsorbed Cr?VI?content in the soil particle interface,and the number of humus reducing functional groups.With the decrease of pH?7.01³.48?,the reduction of Cr?VI?by soil humus was promoted because of K_d lifted,the content of adsorbed Cr?VI?in soil increased,and the concentration of H⁺added at the interface.?3?Cr?T?in the soil transformed from an active state to a substable state and a stable state.With the increase of initial Cr?VI?concentration?40⁴00 mg/L?,the content and proportion of Cr?T?in the active state increased significantly;while those in the substable state and steady state improved obviously with the decrease of pH?7.01³.48?.Among them,Cr?VI?is mainly distributed in Sol and HE states,and Cr?III?is mainly composed of OM and FeMn states.The dynamic transformation of Cr?VI?/Cr?III?occurrence forms was mainly driven by liquid film diffusion,intragranular diffusion,and redox reactions.?4?Soil humus played an important role in the retention process and occurrence forms of chromium in black soil.HE contributed more to the retention and occurrence effect of chromium than HU.This was mainly due to the fact that HE was covered on the surface of the soil medium in the form of an adhesive film with a lower degree of polycondensation,a larger specific surface area,a higher content of active energy-bearing groups.Therefore,compared with HU,HE had a strong adsorption,reduction and complexation ability for Cr?VI?.With the decrease of pH,the contribution of HE to chromium retention and occurrence effect is even more significant.