What is your Point of Zero Charge?

For me, determining the pH point of zero charge (pHpzc) seems to be the hardest part of all experiments involving Biochar synthesis. As simple as the procedure is on paper, it is time-consuming in action, particularly if you have several analytes to immobilize in soil and aqueous media.

Illustrating Point of Zero Charge

As a reminder, my project for the semester centers on using Rice Husk Biochar (RHB) as well as its treatment (with Steam + inorganic phosphate, and with Calcium chloride + inorganic phosphate) for immobilizing the heavy metal matrix (Cd, Pb, and As). I am trying to understand the efficiency of untreated biochar (UB) in comparison with the treated versions (BA and BB), in addition to comparing the efficiency of the two treated versions.

Results from this will ideate the scientific community on how much unit metal is adsorbed from a polluted, infertile soil, with respect to a certain pH value.

Do not forget that the presence of phosphorus in water leading to algal blooms, oxygen depletion and perhaps dead zones is referred to as eutrophication. We do not need our drinking water contaminated with phosphorus in all its forms- organic or inorganic!

But, do we need phosphorus in soil?

Yes, because it is responsible for the general health and vigor of the plant. If you have ever heard of NPK fertilizer, the P is the symbol for phosphorus. NPK (that is, nitrogen, phosphorus, and potassium) symbolizes the three macro-nutrients used by plants for growth. If plants do not have enough phosphorus, their growth is stunted or truncated, and this causes leafy changes. The plant is not able to produce flowers, seeds, fruits and anything that indicates fertility of soil.

To absorb phosphorus, most plants need a soil pH within the range of 6.5 to 6.9. The presence of organic matter and soil organisms also promotes the availability of phosphorus.

What is the significance of the pHpzc in Biochar experimentations? 

I like how the researcher, Alain Celzard of the University of Lorraine, explains it. The point of zero charge is the pH at which the surface of your adsorbent is globally neutral. This means that your biochar or treated biochar contains as much positively charged as negatively charged surface functions.

Below this value, the surface is positively charged; beyond this value, the surface is negatively charged. It is always easier to adsorb a cation on a negatively charged surface, and the opposite is true. But sometimes, other interactions may be stronger than purely electrostatic forces, making the effect of surface charge insignificant.

In 8 separate 50 ml polypropylene tubes, I measured about 0.1 g of untreated Biochar (call it UB). Then I added 10 ml of 5ppm Pb solution. This procedure was repeated for the two treated Biochar samples, making a total of 24 tubes containing a specific contaminant. Since I am also going to adsorb As and Cd subsequently, I repeated the procedure, resulting in a total of 72 polypropylene tubes. 

Each group of 8 tubes for each biochar sample were labeled additionally labelled 1, 2, 4, 6, 9, 11, 13, and 14, denoting the desired solution pH to be adjusted with the aid of NaOH and HCl solutions. 

After recording the initial pH, I am supposed to shake for 24 hours in a mechanical shaker at room temperature, but you know what? I kept the samples latter use (that is, the slurries will be filtered, and the pH of the filtrates will be measured. The pHpzc will be obtained by plotting values of pH change against the initial solution pH).

I have CUMES on Monday, and if you have read my previous post on Opportunity vs Priority, you would know why I have postponed research.

So, what's my point of zero charge? 

Right now, I don't know.