Numerical Simulation Of Heavy Metals In The Reactive Transport In Porous Media

In order to fully study the effect of different physical and chemical factors on the transport of heavy metals in soil,this dissertation studies on four heavy metals which were conducted in small columns full with low-reactive quartz sand under control conditions to obtain precise data and exclude the effect of unknown aspects.On controllable condition in lab,each column was added single or multicomponents of heavy metal ions and the influences of different pH,flow velocity and ionic strength on these ions transport were studied.The source and sinks term,sensitivity,data error and dispersion coefficient sensitivity for one-dimensional advection-dispersion equation(CDE)were analyzed through gradient-regularization algorithm.The analysis indicated that if the data error of experiment was less than 5%the numerical solution accorded with the true solution very well,if the data error was more than 10%,it would affect the simulation.Also,the data of BTCs affected the precision of the simulation.To each experiment,traditional solute transport without chemical reaction was simulated by computer program CXTFIT and coupled chemical reaction solute transport was simulated by the hydrologic geochemical software PHREEQC.The results revealed:Breakthrough time and arrival of peak concentration became earler with the flow velocity increased from 0.05 m/h to 0.3 m/h.The capacity of heavy metals adsorbed on quartz sand decreased in low pH conditions with high peak concentration.There was also earlier breakthrough with the similar BTCs with the ions strength increased.It was well accorded between CXTFIT and PHREEQC in simulation of single heavy metal because the simplification of chemical reaction.The simulation coupled adsorption (with PHREEQC)was better than non-coupled one(with CXTFIT)and it could describe experimental results successfully because the PHREEQC software took into account the reactions of adsorption and ion exchanging.The simulation of the heavy metal Cu²⁺,Zn²⁺,Cd²⁺ was better than the simulation of the heavy metal Pb²⁺because of the difference of the adsorption.The peak concentration significantly increased in multicomponents condition compared with single component condition,which also indicated that the increased ions strength could facilitate heavy metals transport and decrease retention in columns.It had a peak concentration that exceeded the input concentration(C/C₀＞1.0),especially in Cd solution.It indicated that the hydrolysis constant was the important effactor for the chromatographic or snow-plow effect.The chromatographic effect also affected the simulation for the experimental data.The simulation in the high salt(0.25mol/l)and high pH(pH=7.5)condition accorded well with the experimental data.The simulation of solute transport was coupled with chemical reactive such as adsorption and ions exchange,which could reflect the real conditions and provid references for the evaluation and remediation of contaminated soils by heavy metals.