Dear Shyam,

Very interesting since as far as i know the pH under pHzpc makes surface to be positively charged and then would increase the sorption process of anionic substances, am i right? In your case it seems that you have an opposite behavior.

What do you mean by adsorption? Which is the indicator you are using? The reduction in concentration? The sorption capacity?

Perhaps i can try to help in some way.

Hope you get your answer soon.

My best,

Fabio

It might help if we knew the pKa of the anionic substance

true, pzc and pKa should be jointly analysed. Additionally, very close pH values to pzc might induce local pzc fluctuations on the substrate, leading to repulsion (thus adsorption drop)

Dear Shyam,

It is known that before point of zero charge surface of AC is +ve and so adsorption must raised till point of zero charge then start to decrease after point of zero charge where, it start to be -ve charge where repulsion occurs

I believe that you have an error in measuring concentration of the dye during pH changes where dye maximum(lambda maximum) absorption changes with pH change(i.e analytical error) 

Dear Dr. shyam

Do you measured your dye maximum adsorption wavelength at all studied pH?

Thanks for the replies...  Allow me to explain further.  

The substance being adsorbed is an inorganic anionic molecule. 

Upon testing at pH of 2, and 4, and 7 there is a reduction in adsorption in an almost linear fashion.  Adsorption is measured by testing the remaining concentration of the anionic substance in the solution.  Testing at pH of 9 shows a spike in adsorption and at pH 10 there is a decline in the adsorption.  If i omit the result at pH 9, i can draw a straight line to connect the points at pH, 2, 4, 7 and 10. 

Repeated tests at a different temperature yielded the same phenomenon. These were more prominent at 80 deg C , less prominent at 60 deg C and not noticeable at 40 and 24 deg C. 

Surprisingly the adsorption of the anion at pH 9 was better than the adsorption at pH4 at 60 and 80 deg C. At 24 deg C and 40 deg C there was greater adsorption at pH4 rather than at pH9.  

Very interesting observation, Dr. Lakshmanan,

I have a hypothesis. I will assume that adsorption is correctly measured. The adsorption decrease at pH 10 is understandable; it is probably related to uonization of "phenolic" groups. But I would hypothesize that at a certain intermediate pH (9) there is a removal of some impurities, pre-adsorbed rganic material or just organic residuals. They may block some pores or sorption sites but become better soluble at basic pH, thus increasing sorption of inorganic anion. Perhaps, it is easy to check by measuring TOC of activated carbon exposed to pH 9 or by pre-washing the carbon adsorbent by high pH solution, then decreasing pH and carying out all the sorption cycles at various pHs. Could this hypothesis work?

(one more "sub-hypothesis" - ius there any salt bachground or multivalent cation? Ca(2+)? The possibilites for Ca bridging to ionzable OH groups at pH 9?

Thank you Dr Borisover,

The AC was not washed prior to testing. They were tested as received.  There is sodium chloride present in the solution and the anion being adsorbed is chlorate.

Calcium and other cations would  only be present in very low concentrations, at ppb levels. 

I like your hypothesis, and think this may well be the cause. I will test it out.  Thank you so much. 

Thank you Dr Borisover,

The AC was not washed prior to testing. They were tested as received.  There is sodium chloride present in the solution and the anion being adsorbed is chlorate.

Calcium and other cations would  only be present in very low concentrations, at ppb levels. 

I like your hypothesis, and think this may well be the cause. I will test it out.  Thank you so much. 

Dear Shyam,

It is spected that the surface of AC was positive when the pH

Guys I need your help on this.  I have repeated these tests and they keep happening.  Can you tell me if the explanation given below would be a plausible cause for the removal of this polar anion?  Or is any other possible reaction occurring.  Your valued input is welcome.

Since the anion to be adsorbed by the AC is negatively charged, it would be attracted to a positively charged surface of the AC.  However, it was observed that when tested at high pH of around 10, significant amounts of the anion was adsorbed.  Although the pHzpc of the AC was ranging between 8-9, high adsorption of the anion was detected at pH 10, which is above the pHzpc.

At the high pH, my question is whether the highly positively charged ion in the anion molecule could be attracted to the negatively charged surface of the AC, which results in high adsorption of the anion by the AC. 

e.g.  NO3-  is an anion which will be attracted by positively charged AC surface.  The N in the NO3- anion has a (+5) charge.  Would it be possible that the rise in solution  pH which makes the AC surface have a negative charge, would then result in a reorientation of the molecule and greater attraction to the N5+ ion of the NO3- anion.  This then results in greater apparent adsorption of the anion as a whole.

Let me know your thoughts