Should conductivity decrease for mixtures of water and PEG after exposure to atmospheric oxygen? Oxygen should be able to attack the ether bonds in PEG and give rise to increased conductivity, instead of lowering it.
PEG can be an abbreviation for many substances. Do you mean Polyethylene glycol?
pH went from 5.21 to 4.79 after four days but conductivity decreased from 1600uS/cm to 1400uS/cm.
The results seem interesting and a global approach (pH, TSS, etc.) is not enough. Perhaps FTIR or HPLC could give you the answer. Considering ther pH range, it can be linked to the dissociation constants (Ka) of the innitial and final products. Still, the increase in the conductivity is rather high, therefore perhaps you should also check on which are the cations innitially existent in water and which is the variation in water hardness.
the PEG content in the wastewater is about 0.02 - 2% , but we are not sure of the cations and anions inside, probably stainless steel components since its used for the washing of stainless steel machinery.
You have mentioned conductivity was decreased from 1600 microS/cm to 1400 microS/cm. Whether the initial conductivity value (1600 microS/cm) refers to cond of waste water before adding PEG or it is just after adding PEG? If it is just after addition of PEG what is the conductivity of water before addition of PEG?
The PEG increases the viscosity of solutions. The increase of viscosity should reduce the value of the conductivity (the mobility of ions is reduced in a viscous solution). You should maybe check this point.
conductivity of 1600uS/cm was already added with PEG , the next 1400uS/cm reading was taken 4 days later.
It may be possible to predict that due to oxygen attack to PEG bond the conductivity may increase and pH may decrease. But effect of the medium also plays a role. By addition of PEG the overall viscosity of the medium decreases from that of the solution medium before addition of PEG. In order to confirm this statement if you add 1 mL of glycol to tapwater you will find that the condcyivity of tap water decreases. Thus it is clear that the viscosity of the medium plays a key role to decide the shift in conductivity.
Thanks for the explanations!
The conductivity observed was on the same wastewater on the first and fourth day, so viscosity is the same for both samples. Therefore it might be other factors that has lead to a observed decreased in conductivity.
You mentioned that you have some iron cations in your water. Typically, these tend to hydrolise in time, forming hydroxides that form colloids but, what is more important, they are not dissociated thus their formation decreases conductivity. Now, this should happen with and wihtout PEG but the reaction rate can be different with PEG addition. You should check conductivity evolution in a reference sample (without PEG).
The iron hydroxide formation, even in low concentration can be eye-sensed as the water gets a slight yellowish shade and is slightly blurred. Of course, there are plenty of analytical investigations that can give evidence in a quantitative way
In order to cofirm formation of colloidal iron hydroxide in solution immediately after addition of PEG real time monitoring of conductivity is the best choice. From online conductivity plot one can get some information regarding kinetic of the reaction which may be useful for better explanation of decrease in conductivity as you experienced.
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