Results after 24hours shows minimal (if any) removal eg. 25ppm (initial) to 23ppm (after 24hrs) for Pb,Cd and Zn performed at 22oC and pH 2. Literature still suggests more adsorption to take place than I have found.
Biochars used were higher temperatures (550/700oC) and showed smaller amounts of functional groups due to higher pyrolysis temperature
Could it be that competition between Metals ions and H3O+ at low pH is causing poor adsorption to active sites?
My next step would be to look at different pH as I am aware this is relatively low for peak adsorption of metals analysed but I am still unsure as to why my results are so low compared to literature.
Thank you for any advice.
Natalie , have you analysed the biochar for physico-chemical properties , please have a relook at those properties to ascertain the quality of biochar .
In general, sorption at low pH is quite low due to the increased protonation of functional groups. So the functional groups may become neutral if they were deprotonated before (e.g. carboxyl) or gained extra H+ and become positive (e.g. amine).
would seem reasonable to do measurement at or close to neutral pH. Obviously at alkaline there will be precipitation of metal hydroxides.
As per the provided information, your biochar will be alkaline and possibly have higher surface area. It can be verified by physical analysis and BET.
For low adsorption efficiency, there might be fusion of the material due to high temperature, I guess.
Thank you all.
I already have results for physio-chem properties (in relative units):
550
Moisture 1.52
Ctot 85.52
H 2.77
O 10.36
H:C 0.39
O:C 0.09
Total Ash 1.25
Ph 7.91
Total Surface Area 26.4
700
Moisture 1
Ctot 90.21
H 1.83
O 6.02
H:C 0.24
O:C 0.05
Total Ash 1.89
Ph 8.44
Total Surface Area 162.3
Q (mg/g) for Pb2+ are approx 2.7 and 10 depending on initial concentration /biochar used yet I am still confused as to why my Q values are similar to that of past research yet I see very little removal over 24 hours!
How can my adsorption capacity (Q) be so high if so little is removed? (I am getting very low removal efficiency results to support)...
I will repeat the experiments at varying pH levels 2-7 to compare results and see if this improves sorption capacity, thank you all for your advice.
Q is dependent on concentrations, ratios of sorbent to metal, ionic strength, pH, temperature,...
Same Q but little removal? Wouldn't be that your biochar gets saturated and you have too much metal in solution?
How do you measure the removal? By percentage of the initial concentration?
I've been using the equation: Removal % = (Ci-Cf/Ci) *100
Where Ci is initial conc
Cf is final conc
I'm not sure I understand, what would the biochar be saturated with if little to no removal occurs over the time then surely it can't be with the metal? As even after e.g 30 minutes little to no removal takes place. Thank you!
With metal, but because of the protonation, as said before, the groups where they can attach are very limited.
I had this written somewhere else, but it may help to understand the concepts.
Q and removal % are calculated from the same data. But the sorption capacity is given by the Q not by the removal. They measure metal removal: Q from the point of view of the sorbent and the % from the point of view of the solution.
We can say that the Q is sort of a measurement for how many functional groups are occupied or can be occupied in the sorbent, but the removal refers to the metal that is missing from solution after incubation.
e.g. 1 g biochar incubated in 100 mL solution of 10 g/L metal. After 1 h of incubation, the concentration is 9 g/L.
Based on that, the removal would be 10% and the Q would be 100 mg/g. It would be possible to say that in those conditions the sorbent is saturated, given that most of the metal remains in solution.
Hypothetically if you would increase the volume used,e.g using 1 L solution instead of 100 mL and leaving all the other parameters intact, the removal would drop to a 1% but the Q would still be 100 mg/g. Off course this would be debatable, as the ratio of sorbent/solution would be 10 times lower.
Dear Natalie
I would say that you have to measure ZPC of your biochar. I think your pH is too low for CEC to occure. ZPC is value of pH for which the total net charge σT is cancelled (σT= 0) . Below this point surface of biochar will be dominated by positive surface functional groups and there will be possible anion sorption and above the pH pont higher thatn ZPC surface charge will be dominated by Negative charges. This will increase CEC of the soil. Which means that above this pH point soption for your metals (Pb,Cd and Zn) will increased. But it does not mean that below ZPC pH there is no sorption of Cations but it may decrease as pH decrease below this pont. Thank you.
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