Impact And Control Mechanisms Of The Geochemical Environment On The Solidification Of Chromium In Soil

Solidifying of chromium insoil is a process that mobile fractions transform into relatively immobilized fractions after the entrance of exogenous chromium into soil.Bound fractions of chromium in soil showed different stabilities,toxicity and bioavailability,so comparing to valence,the bound fractions of chromium more veritably reflected realistic residence fraction.And remediation technologies s at the present were difficult to ensure the long-term stability of the remediation effect.resulting in the secondary pollution of the soil.or changed physicochemical properties of soil with decreasing soil quality.Under the condition of unchanging physicochemical properties of soil,reasonable changing geochemical environment decreased secondary pollution of the soil and enhanced inertia of chromium in soil.Therefore,research on transformation mechanism of chromium fraction in soil under natural geochemical environment was conducive to understand the geochemical cycle of chromium and its impact on the ecological environment in order to scientifically assess the ecological risk of and human health risk of soil pollution with chromium.This study investigated by submerged incubation experiment:(1)the natural solidification process after exogenous chromium entering into soil under unchanging physicochemical properties of soil:(2)the effects of water content,temperature variation and particle size on the solidification of chromium in soil;(3)solidification process of chromium in soil adding DOM and humic acid,adding iron-manganese composited oxide and iron oxide,compared to original soil system.The main conclusions were as follows:(1)The solidification process of exogenous chromium in soil was divided into early fast stage and late slow stage with sdsorption and diffusion.The rapid transformation of fractions of chromium in the early was mainly driven by the concentration difference of chromium ion on the outer surface in soil and the water-soil environment.The slow transformation of fractions in latter was the gradual infiltration and diffusion process of chromium ion into the micropores in soil.The solidification process of chromium was shown as the transformation from the mobile fractions to immobilized fractions.Solidification effect continued at the end of the incubation experiment.It showed that the solidification process of chromium was a long and slow process.(2)Water content influenced the fraction transformation of exogenous chromium in soi by changing the soil pH.Eh.carbonate and organic matter content.The higher water content enhanced the adsorption capacity of heavy metal ions on the adsorption material,but also hindered other fractions transformation of chromium to the residue fraction,so reduced the effect of solidification.The higher water content enhanced the solidification effect and effectively reduced the groundwater pollution and ecological risk of exogenous chromium.(3)The increase of temperature accelerated the ion diffusion rate in the soil,which promoted the transformation from the mobile fractions to relatively immobilized fractions and enhanced the solidification effect,then the groundwate pollution and ecological risk of exogenous chromium in soil was smaller.In the low temperature environment,especially the freez-thaw environment,the residual fraction of chromium had the risk of activation,then the groundwater pollution and ecological risk of exogenous chromium in soil was higher.In the different particle sizes,fine particle enhanced the solidification effect which reduced groundwater pollution and ecological risk.(4)Adding iron-manganese oxides and iron oxides enhanced the solidification effect of chromium in soil and reduced groundwater pollution and ecological risk.Comparing adding iron-manganese oxides,the solidification effect of chromium was higher adding iron oxides;in contrast,iron-manganese oxides had stronger oxidation ability and activity.(5)Adding DOM and humic acid had little effect on the solidification process of chromium in the soil.It slightly increased of the groundwater pollution and ecological risk of exogenous chromium in the soil and only reduced ecological risk in later.The solidification process of chromium in the two organic systems were different.DOM promoted the transformation from the mobile fractions to the iron-manganese oxides and residue fractions;humic acid promoted the transformation from the mobile fractions to the iron-manganese oxides and organic high bonding fractions.(6)Water content and temperature were the main controlling factors of chromium solidification in the study area.Taking certain measures such as increasing temperature at a low temperature,drainage at rainfall or irrigation effectively enhanced the solidification effect of chromium,so as to reduce the risk of groundwater pollution and ecological.