Hello, I would appreciate any help with the following:
My experiments are currently focusing on metal adsorption by biochar materials at different pH. When making metal solutions (e.g. Potassium chromate or Lead nitrate), research papers tend to say, 'pH was adjusted with' but do not state at which point this is carried out.
My typical procedure has been:
1) Make metal dilution from stock, and add water to nearly 1Litre (~900mL)
2) Adjust pH using 0.1M acid or base, adding this dropwise.
3) Top up to 1L.
4) Decant into vessels (50mL tubes)
5) Add biochar and begin the experiment.
However, once the biochar is added, the liming effect occurs, increasing the pH. This changes depending on the feedstock, thus making a comparison between the different biochars difficult.
My question is: Does the adjustment of pH in batch adsorption experiments occur after the biochar is in the batch vessel (centrifuge tube, glass flask, etc) with the solution i.e adjusting each vessel in the batch? ** if so, does this not effect dose and concentration when comparing different feedstock materials?**
OR
At the point of making the dilution?
If neither are correct, please could someone kindly suggest an alternative methodology?
Thank you
Dear Dr. Natalie
I think you should adjust the desired pH after adding the adsorbent materials into the batch vessels especially if the adsorbent you use contains functional groups which may immediately affect the pH of the media , so if you fear from the initial concentration change, you can use more concetrated Acids or Bases like 1 M for example. by this way you can prepare you working solutions then add the adsorbent then adjust the pH using concentrated Acid or Base via micropipet.
Dear Natalie, you have two solutions. You can adjust the pH before adding the adsorbent, but in this case you must measure the pH at the end of your experiment because the adsorbent will change its value. However, if you want to compare the adsorption properties of your adsorbents at the same pH, i think that you need to adjust the pH just after the contact between adsorbent and adsorbate. You will dilute a little your solutions, but if you use concentrated solution, it will be negligible. Moreover, don't forget to measure the pH value at the end of your experiment, because some adsorbents can strongly buffer the pH with various kinetic.
Regards
Hi Natalie,
Couple of thoughts: You could prepare pH solutions using reagent water that is free of metals that you are measuring (always test your reagent water for metals). Add this to biochar in 50 mL tubes, allow time for pH to stabilize, measure and adjust pH to desired value, then once the pH has stabilized spike each 50 mL centrifuge tube with metals to desired concentration. The easier way would be to just do what you are doing currently and measure pH at specific intervals and include that data in your research narrative. It would actually be a pretty good chart to incorporate in your paper. If you want to be very precise, you could pre-condition large batch of biochar at pH's of interest with metal free reagent water at pH of interest. This would minimize fluctuation of pH you are seeing at the beginning of your experiments.
Good Luck!
Never adjust the pH of adsorbate solution after adding the adsorbent specially in the case of biochar because when you do this you are going to destroy the surface property of the biochar. Firstly, you have to adjust the pH of DDW or your water sample and then add the desired metal. Now you can add the desirable dose of biochar and keep the sample in incubator shaker. One more thing, you must need to analyse the pH value of the solution after sorption at the time of batch experiments.
All the best.
Regards
Thank you all for the helpful advice. I will repeat the experiment making the necessary changes and hopefully see better results.
kind regards
According to my experience you should adjust the desired pH after adding the adsorbent materials into the batch vessels especially if the adsorbent you use contains functional groups which may immediately affect the pH of the media , so if you fear from the initial concentration change, you can use more concentrated Acids or Bases like 1 M for example. by this way you can prepare you working solutions then add the adsorbent then adjust the pH using concentrated Acid or Base using droppers. Thanks
Hi Natalie,
Some tips might be useful to you:
Take into consideration that if the pH goes higher during the adsorption, metal may form complex with OH-, for example, Pb(OH)2, Pb(OH)3− and Pb(OH)42− at higher pHs . Lead hydroxyl species may participate in the adsorption and/or precipitate onto the adsorbent (Characterization of Minerals, Metals, and Materials 2017. Springer International Publishing, 2017. 91-100.).
If your adsorbents change the pH of solution by liming effect, you can try using lowest amount of adsorbent for your batch study to avoid changing the pHs. Then you have to decrease your initial metal concentration as well based on your adsorption capacity that you expect.
Good luck with your experiment!
Mahmoud
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