Measurement Of Labile Cadmium In Waters With DGT Technique Using PAAS As The Binding Phase

The diffusive gradients in thin-films technique (DGT) offer a kind of new method which can measure some active material in solution in situ. DGT uses a cellulose dialysis membrane or hydrogel to separate binding phase and bulk solution, to control ionic exchange and accumulate in situ. DGT mostly be used for a kind of monitoring instrument of pollution or bioavailability diffusive flux in situ mensuration.In this article we used liquid state Sodium polyacrylate (PAAS) as binding phase, cellulose dialysis membrane (CDM) as diffusion layer, the CDM-PAAS-DGT device can accumulate labile Cadmium in solution effectively. DGT devices do not measure all the chemical species of a metal in solutions. It only measures those species that can be accumulated by the DGT binding phase which is one of the advantages for DGT.This paper first studied whether the CDM-PAAS-DGT device had the ability to measure Cd2+, Co2+, Ni2+ and Zn2+. The experiment results revealed that the CDM-PAAS-DGT device only had the ability to measure Cd2+ but not Co2+, Ni2+ and Zn2+. Between the PAAS and Cd2+ exsit a strong bonding ability with the condition stability constant 10.5 which can give us the information that PAAS and Cd2+ can bond strongly. PAAS solution had the strong ability to bond labile Cadmium with other metals coexisting.3.0 mmol/L PAAS solution was selected as the better binding concentration. A deep study for Cd (Ⅱ) was carried out in this paper and the experiment results showed that the measurement was independent of pH in the range of 5-8 and the accumulation ability decreases with the increasing electrolyte concentration.2.0 mL 3 mmol/L PAAS solution can accumulate Labile Cadmium in waters up to 4.98μmol。The paper also studied the application of CDM-PAAS-DGT devices in the mixed synthetic waters and natural waters. In the mixed synthetic waters the recovery for Cd (Ⅱ) can reach 92.2% which illustrated that the main speciation for Cd (Ⅱ) is ion in the mixed synthetic waters. On the contrary the recovery for Cd (Ⅱ) was very low in the Nanhu River only 21.3% and lowest in the industry waste water. This is because a lot of strong ligants exist in the natural waters which can bond Cd2+ stably, as a result, the labile cadmium for DGT becomes fewer and fewer.