A Simplified Solute Transport Model And Experimental Study Of The Efficiency Coefficient Of Clay Membranes In A Reverse Osmosis System

The traditional hydrogeological theory is based on groundwater head pressure as the only driving force of groundwater flow to establish a groundwater flow model,applying the groundwater flow model to the convection-dispersion equation which describes the law of solute transport,and get the law of solute transport.For the study of solute transport in an aquitard consisting of cohesive soil with a semipermeable membrane characteristic,however,it is necessary to consider the effect of the semipermeable membrane effect of cohesive soil on solute transport.Traditional hydrogeological theory is difficult to be applied to this situation because the water drive force in the aquitard is not only the head pressure,but also the osmotic pressure.This paper basing previous studies derived the simplified model of solute transport in cohesive soils and simulated the indoor test of the reverse osmosis system in a given flow rate and infiltration end concentration by using the model.In this paper,we first determined the mass flux expression of solute from the perspective of non-equilibrium thermodynamics,and selected the typical unit body in the cohesive soil.According to the principle of mass conservation,the solute migration model was deduced.The simplification of the solute transport model was discussed from the aspects of chemical permeation and reverse osmosis.The indoor test of chemicalpenetration could control the permeability conditions,making the infiltration system being almost no flow state.We inferred that concentration of solute in clay changed with time depending mainly on the molecular diffusion,and the changes of concentration caused by fluid flow could be ignored.The simplified model of reverse osmosis was based on the fact that the clay layer was the channel of the solvent.The fluid flow in the clay was considered to be time-dependent and position-independent.When we used the model,the interaquifer flow in the reverse osmosis system was required to be much greater than the amount of "stored" water of the clay layer at the same time or the thickness of the clay layer could meet the requirements.Subsequently,we designed a reverse osmosis laboratory test with a given flow rate and infiltration end concentration,and the coefficient of clay film efficiency was calculated by the analytic solutions under experimental conditions deduced by using the simplified model of reverse osmosis.We analyzed the reliability of this method.The results showed that the efficiency coefficient of remolded clay film was 0.9010-0.9064 under the experimental conditions,indicating that the membrane performance of clay was good.However,the osmotic concentration reached 0.0998 mol·L-1,close to the infiltration concentration of 0.1 mol·L-1 after 120 days,indicating that the ability of the clay membrane to intercept salt decreased with time.In addition,the sensitivity analysis in the quasi-steady state showed that when the flow rate assumed value was within ±10%of the original value,its effect on the membrane effect coefficient did not exceed ±1.25%and the effect of quasi-stable length on the membrane effect coefficient was not more than 0.71%,which indicated that the calculation method had high reliability.Finally,we discussed the effect of infiltration concentration on the membrane coefficient of cohesive soil.The membrane efficiency coefficient value was adjusted,and then adjusted the effective molecular diffusion coefficient value according to the relationship of the effective molecular diffusion coefficient and membrane efficiency coefficient.We used the finite element numerical simulation software,comsol,to simulate and calculate exudation concentration data of the second reverse osmosis test and the result showed a fact that the calculated results was more consistent with the testresults when the infiltration concentration increased and the value of the membrane efficiency coefficient of the second reverse osmosis test was smaller than the first reverse osmosis test value,The experimental phenomena indicated that the membrane efficiency coefficient of cohesive soil decreased with infiltration concentration increasing.