Effect Of Soil Colloid And Preferrential Flow On Cadmium Behavior In Soil

The risk of leaching of strongly adsorbing contaminants had been assumed to be low, but such compounds have been reported to move large distances in soils and sediments. This has motivated research towards facilitated transport and preferential flow, two mechanisms that could enhance transport rates. Laboratory column studies have demonstrated co-transport of contaminants sorbed to suspended colloids, or simultaneous leaching of in situ colloids and contaminants, while field studies have revealed the association of contaminants with colloids in drain or groundwater. Facilitated transport and preferential flow are interrelated because colloid transport velocities are higher in macropores, where preferential flow occurs. While the direct effect of preferential flow on the transport of colloid and the colloid-facilitated contaminiants transport is unavailable. The objectives of this research were:1.To determine the general contribution of colloid on the adsorption of Cadmium in different soils;2.To determine the effect of colloid on the bioavailability of Cadmium to plant;3.To determine the effect of colloid on the transport of Cadmium in a repacked soil column with quartz veins;4.To determine the effect of colloid on the transport of Cadmium in field soil monolith and the extent to which the distribution of Cadmium in the profile were associated with the observed preferential flow paths.To meet these objectives, batch experiments and pot experiments and soil column experiments and a field experiment were conducted. The main results are as follows:1. Batch experiments were conducted to investigate the equilibrium sorption isotherms for Cd²⁺ onto particle-sized fractions(2000-200μm, 200-20μm, 20-2μm and 2-0μm, which were named coarse sand, fine sand, silt and clay according to international system respectively) derived from four typical soils(black soil, yellow brown soil, huangnitu paddy soil, and red soil) in China by the use of a novel technology combined with wet sieving, sedimentation-siphoning and centrifugation method. The results showed that adsorption of Cd²⁺ by these fractions could be described by the Langmiur equation Ce=Xm·Ce/X-k^-1 and Frendlich equation X=Kd·Ceⁿ. The maximum adsorption of Cd²⁺ (Xm) and distribution coefficient (Kd) decreased in an order: clay>silt>coarse sand>fine sand in the four soils. The average maximum adsorption of Cd²⁺by the clays of the four soils was 202.8±64.6mmol·kg^-1, and the maximum adsorption of Cd²⁺ by silt and fine sand and coarse sand were 135.0±39.2 mmol·kg^-1, 47.0±9.4 mmol·kg^-1 and 81.3±33.7 mmol·kg^-1 respectively. Accounting to the contents of the fractions in the soils, the contribution of each fraction followed by the order: silt (52.2±15.3%)> clay (32.3±14.9%) > fine sand (13.8±3.4%)> coarse sand (1.7±1.4%). The percent of Cd²⁺ adsorbed by silt and clay of the four soils exceeded 80%. By choosing particle size, organic matter, iron oxide and equilibrium pH value as parameters, SPSS software and Multivariate Statistical Analysis were employed to build the regression model of adsorption of Cd²⁺ and to evaluate the effect of these factors on it. The results showed that particle size, organic matter and equilibrium pH value significantly affected the adsorption of Cd²⁺. But iron oxide had no significant effect on the adsorption.2. A batch method was used to investigate the kinetics and thermodynamics of cadmium adsorption onto different particle-sized fractions derived from yellow brown soil by using the same method as described above. The results showed that the reaction of adsorption can be divided into two types: a fast reaction in the first 15min and a slow reaction in the later reaction course. The amounts of Cd²⁺ adsorbed by the fast reaction exceed 95% of the adsorption capacity. As the temperature increased from 25℃to 45℃, the adsorption capacities of the four fractions increase by 4.86%-25.3%. First-order rate equation and pseudo second-order rate equations were applied to express adsorption kinetics. Adsorption processes for Cd²⁺ onto the four fractions are found to follow pseudo-second order type adsorption kinetics. The pseudo second-order rate constants exhibited that adsorption speed to reach equilibrium decreased with the increase of particle size. Intra-particle diffusion might be the major rate-limiting step. Thermodynamic parameters includingâ–³H°,â–³S°andâ–³G°were also calculated from graphical interpretation of the experimental data. Positiveâ–³H°values indicated the adsorption processes to be endothermic. Negativeâ–³G°values implied that adsorption reaction was a spontaneous process.3. A greenhouse pot experiment was conducted to investigate the bioavailability of Cd of different soil matrix (colloid, raw soil and de-colloid soil, derived from yellow brown soil and red soil and artificially contaminated by Cd with a concentration of 10.91mg·kg^-1) to ryegrass and the effect of EDTA on it. A laboratory batch experiment was also conducted to study the potential mechanism involved it. The results showed that: (1) the mean shoot height, shoot dry weight, root weight and the total biomass were in the same order of colloid > raw soil > de-colloid soil. The total biomass in the colloid treatment was 1.31±0.02 and 1.82±0.21 times over that in raw soil and de-colloid soil respectively. (2) Cd concentration and bioaccumulation factor of shoot and root were followed by colloid < raw soil < de-colloid soil, indicating the bioavailability of Cd to ryegrass was colloid < raw soil < de-colloid soil. (3) EDTA addition (disodium salt, 27.27 mg·kg^-1) led to significant increase in Cd concentration of shoot and root and decrease in biomass of ryegrass. (4) Batch experiment revealed that the activation effect magnitude of EDTA on Cd was followed by de-colloid soil > raw soil > colloid, and this effect was influenced intensively by the initial pH value.4. A laboratory soil column experiment was conducted to investigate the artificial macropore on the colloid and DOM mediated Cd transport. Compared with the control treatment with no macropore in the soil column, macropore treatment enhanced leaching by 75%. The addition of colloid and DOM increased the loading of Cd by 16% and 50% respectively.5. The potential role of soil colloids and DOM in transporting Cd was investigated through in situ undisturbed paddy soil monoliths. In addition, Brilliant Blue FCF was used as a tracer to assess the effect of preferential flow on Cd down migration. Soil colloid fractionated from the paddy soil and DOM from rice straw were spiked with 5 mg·L^-1 of Cd, and then applied onto the surface of two soil plots (1m×1m) to maintain 150 mm water depth after 72 hour equilibration for Cd²⁺ with soil colloid or DOM. The solution containing Cd but without the addition of soil colloid and DOM was also applied as control treatment. One day later 100 L of dye solution (1g·L^-1) and sequent 100 L of tap water were applied to the plot. After the infiltration was completed, the vertical profiles excavated in the plot were observed for dye movement or preferential flow. Soil samples collected from homogeneously stained and unstained areas with different dye intensities at the various depths were determined for Brilliant Blue, water soluble Cd, exchangeable Cd, and total Cd. The results obtained from classic statistical and geostatistical analyses showed that deep penetration of Cd and Brilliant Blue into the soil profile took place due to preferential flow through macropores, mainly earthworm channels, with much of the chemicals thus bypassing the soil matrix. Dye tracer and Cd distribution within the soil matrix was fairly restricted to several centimeters. Colloid restrained the migration of dye and Cd both in the matrix and preferential flow area. DOM facilitated the transport of Cd and Brilliant Blue in matrix and macropores by about 10 cm over that of the control. Pearson correlation analysis revealed strong associations between Brilliant Blue concentrations and exchangeable and total Cd concentrations in the three plots indicating that they had taken the same preferential flow pathway.