Effective Concentration Difference Model Of Diffusive Phase In The Diffusive Gradients In Thin-films Technique And Its Application

The diffusive gradient in thin-films (DGT) is a technique that has been used to the measurement of labile metals in waters, soils and sediments. The new reported liquid binding phase DGT device has two main advantages over the solid binding phase DGT device. Firstly, the liquid binding phase can contact closely with the dialysis membrane in the liquid binding phase DGT device, and the potential imperfect contact between the binding phase and the diffusive layer in solid binding phase DGT was overcome, which resulted in the increase of analytical accuracy. Secondly, the DGT procedures were simplified by the removal of elution steps required for all solid binding phases, which resulted in the increase of analytical precision. The only drawback is the need to dilute and matrix match standards to the solution for analysis in the measurement of diffusion coefficient, as the diffusion coefficients in the dialysis membrane are associated with the composition of source solution and receptor solution. In this paper, we study systematically the effect of the composition of source solution and receptor solution, such as ionic strength, binding agent, ligand and Donnan potential, on the effective diffusion coefficient; simplify the procedure of determining a diffusion coefficient to allow the application of liquid binding phase DGT devices; determine the optimum binding agents of both sodium polyacrylate (PAAS) series and poly (4-styrenesulfonate)(PSS) series in the measurement of heavy metal, and compare DGT technique with ion elective electrode.In chapter1, the in situ sampling and measurement methods including dialysis peeper, diffusive equilibrium in thin-films (DET) technique and DGT technique were reviewed. The principle, construction, diffusive phase, binding phase and application of DGT technique were summarized. The trends of DGT technique were evaluated and the objective and innovation of this paper were presented.In chapter2, the effective concentration difference model (ECDM), based on the assumption that the effective diffusion coefficient of metal ion in the dialysis membrane is decided by the effective concentration difference on the both sides of the dialysis membrane, is proposed and applied to study the effect of ionic strength, binding agent, ligands and Donnan potential on the effective diffusion coefficient. In chapter3, ECDM was tested by measuring the effective diffusion coefficients of Cd2+in the dialysis membrane under various conditions. The effect of ionic strength, binding agent, ligands and Donnan potential on the effective diffusion coefficient was observed in the experiment, and ECDM can be applied to explain and analyze successfully the effect of various factors on the effective diffusion coefficient.The method of determining a diffusion coefficient was improved under the direction of ECDM. Studies showed that the effective diffusion coefficient works in the application of liquid binding phase DGT only when the determination of effective diffusion coefficient and its application are in the same environment. The simplified procedure of determining a diffusion coefficient is as follows:the effective diffusion coefficients of metal ions in the dialysis membrane are measured in a diffusion cell with binding agent deionized water solution as receptor solution and spiked natural water as source solution in the application of DGT with liquid binding phase. The DGT labile fraction, calculated with the effective diffusion coefficient through the dialysis membrane immersed in the deionized water solution with binding agent as receptor solution and in the spiked natural water as source solution, is the actual DGT labile fraction. The DGT labile concentration, calculated with the effective diffusion coefficient through the dialysis membrane immersed in the deionized water solution with binding agent as receptor solution and in the spiked synthetic water as source solution, is equal to the free ion concentration determined by ion selective electrode. The results showed that DGT could measure selectively the free ion concentration in water samples. Moreover, the in-built metal pre-concentration procedure of DGT allowed a metal ion to be measured at concentrations below the detection limit of a direct determination by ISE.In chapter4, poly (4-styrenesulfonate)(PSS) with a relatively high molecular weight of1×106(PSS-E6), instead of poly (4-styrenesulfonate) with a relatively low molecular weight of7×104(PSS-7E4), was used as the binding phase in the liquid-type DGT device (CDM PSS-E6DGT) to measure heavy metal ions. The recoveries of Cu2+, Cd2+, Co2+, Ni2+in the synthetic lake water were96.53%,97.48%,96.72%and95.61%, respectively, and the DGT labile fraction (Ψ) of Cu2+, Cd2+, Co2+, Ni2+in the spiked river water were26.01%,42.96%,16.41%and18.24%, respectively. The average pretreatment depletion rates (n=5) of PSS-7E4and PSS-E6were70.33±1.22%and15.18±0.87%, respectively. The binding capacities of CDM PSS-E6DGT to Cu2+, Cd2+, Co2+and Ni2+were9.9,10.6,10.1and9.8μmol·mL-1, respectively. The stability constants (logK) of PSS-E6with Cu2+, Cd2+, Co2+and Ni2+were8.5,9.4,8.6and8.3, respectively. PSS-E6is more suitable as the binding phase of DGT for its higher binding ability, lower pretreatment depletion rate, lower cost, and lower pollution. In chapter5, sodium polyacrylate (PAAS) with various molecular weight (Mw～5×104, PAAS-5E4; Mw～1×105, PAAS-E5; Mw～4×106, PAAS-4E6; Mw～2×107, PAAS-2E7) were used as the binding phase in the liquid-type DGT device (CDM PAAS DGT) to measure heavy metal ions. The optimum concentration of PAAS-5E4, PAAS-E5and PAAS-4E6was0.0050mol·L-1, the optimum concentration of PAAS-2E7was0.0030mol·L-1. The binding capacities (μmol·mL-1) of CDM PAAS-5E4DGT, CDM PAAS-E5DGT and CDM PAAS-4E6DGT to Cu2+, Cd2+, Co2+and Ni2+were2.30～2.50, the binding capacities (μmol·mL-1) of CDM PAAS-2E7DGT to Cu2+, Cd2+, Co2+and Ni2+were1.30～1.50. The stability constants (logK) of PAAS-5E4, PAAS-E5, PAAS-4E6and PAAS-2E7with Cu2+, Cd2+, Co2+and Ni2+were7.90～8.62. The uptakes of Cu2+, Cd2+, Co2+and Ni2+by CDM PAAS-5E4DGT, CDM PAAS-E5DGT, CDM PAAS-4E6DGT and CDM PAAS-2E7DGT increased linearly with time up to96h, and the recoveries of Cu2+, Cd2+, Co2+, Ni2+in the synthetic lake water were all above90%. CDM PAAS-5E4DGT, CDM PAAS-E5DGT, CDM PAAS-4E6DGT and CDM PAAS-2E7DGT all can accumulate and measure Cu2+, Cd2+, Co2+and Ni2+in the water. Experiments showed that PAAS-E5was the optimum binding agent of PAAS series in the measurement of heavy metal ions using PAAS DGT devices.