There are many influences on pH in any soil, perhaps sandy soils are one of the simper ones to model. You should specifically look at the organic fraction and salt content (not just NaCl, but nitrate/carbonate/phosphate/sulphate compounds too). Of course, temperature, etc will also influence the pH. However, the best way to find out is to experiment, even a quick experiment passing your solution through a sample of the soil in the lab will tell you a lot. Have you thought about buffering your solution? There are plenty of buffering systems out there for aqueous solutions which may help maintain the pH and may be worth looking into if this is what you want to do. good luck!

Will depend also on the velocity of the flow (mL/min), dissolution rates of salts, amount of solution and soil, temperature, kind of solution (ultra-pure water?), atmospheric CO2 in contact with solution... I recommend using a speciation program such as Visual Minteq to simulate your system.

I would turn the question around and ask what is the pH buffering capacity of your solution. If your solution is composed of simple ionic salts then you can assume minimal buffering capacity other than the dissolved CO2 in the water. As a result, the pH would change to match the influence of the porous matrix. The rate of change depends on the variables described by Dr. Marchi in his answer. I agree that a chemical speciation model like Visual Minteq would be useful to describe equilibrium boundaries for possible pH changes. However, the accuracy of the speciation model will be improved by supplying more information about the solid phase as well assumed partial pressure of CO2 or other gases within the porous medium.

I agree with the comments above. Also, the term "sand" is simply a definition of size of particles. The composition of the particles, and their ability to buffer acidity, can vary widely. X-ray diffraction of the bulk soil sample or its size fractions would help you interpret your experimental results.

Once water perculate the soil it is not dry any more. Since the actuak solution that you can measure is the part yhat exude from the bottom of the soil tube, most of the changes are not measurable. It depend on the salt level in the soil, ca carbonate levels in the water and in the soil, and the length of water perculation. mind that within 24 h micro organsms will start to develop and their respiration will also affect the pH/ Always remember that soil is not a sterile chemical and microorganism activity will depend on the nutrient suplly in the water that will affect their activity, as wekk as temperature effects that influence the living activity of microbs in soil/

Just as above every one said, pH in soil is a dynamic property and it is never fixed even some of them has high buffe capcity. Sandy soil usually has less buffer capacity and sensitive to the conditions of environments, e.g temperature, water content.

Meanwhile, the reactions might not uniform and they have variability from the view of sapace and time too.Time is another factor that you should considered.

It depends on the content of carbonates of that solution:

If there are too many carbonates in that solution(>400mg/L), then it will have too high buffer capacity and then too little changes in pH after soil interaction will happen in the solution... conversely, in case of a solution with very little concentration of carbonates (

It depends on the content of carbonates of that solution:

If there are too many carbonates in that solution(>400mg/L), then it will have too high buffer capacity and then too little changes in pH after soil interaction will happen in the solution... conversely, in case of a solution with very little concentration of carbonates (

Hamed, I recommend conducting a pH titration test on your soil and solution both. This will provide you with direct and quantifiable information on the buffer capacity of your soil. This direct information is much easier to obtain than trying to predict what the change in pH would be. In other words, sometimes it is easier to directly measure something than it is to predict it! It is very easy and inexpensive. Simply titrate with known concentration of acid (or base) with a burette and monitor change in pH with every titration. Then plot the equivelants of acid (or base) added against pH. As many have commented, there are a large number of factors that impact the buffer capacity of soil

Hi hamed, I agree with Chad above, just some addition: there are standard protocols available for measuring the buffering capacity of materials. An example is the ISO/PRF TS 21268-4 test, a test developed for soils at ISO- level, which is basically a titration towards more acid or alkaline conditions. When you have measured the buffering capacity in sufficiently fine resolution (small steps), it is fairly simple to calculate the end-pH by keeping a mass balance of your influent (acidity or alkalinity added) and the buffering capacity of the soil.This approach is more accurate than using (predictive) speciation programs; so far it has been very difficult to predict pH from the individual soil constituents without calibration. Good luck!

Thanks to all for the answers above.

The solution which I am going to inject is composed of Nutrient Broth (3g/L), NH4Cl (20g/L), NaHCO3(2.1g/L), Urea (0.2-1.2M), CaCl2 (equimolar with urea), and different concentrations of an urease active type of bacteria. The aim is CaCO3 precipitation within the soil. The bacteria produce urease catalyzing the reactions resulting in CaCO3 precipitation in the soil. During the reactions the media tend to be basic due to decomposition of urea to ammonium. NaHCO3 acts as pH stabilizer.

The bacteria can not grow in pH less than 6.5-7 and more than 10.5. So the precipitation doesn't happen. The pH of the solution is 6.8-7.5 (depends on different concentration of equimolar CaCl2/Urea) by itself. Now, my concern is keeping the pH in the optimum range between 7-9 after injection into the soil.

looking forward your comments...

I would anticipate that if calcite precipitates, the pH of the soil will be close to 8.0-8.3 and rather invariant, according to the calcite dissolution/precipitation equilibrium. The calcite will act as a buffer. If you add sufficient NaHCo3 and CaCl2, and make sure the pH of the solution is not too low (I would start with pH 7.5) I would not be very worried that the pH will run out of the window pH 7-9. I think the best advice now is: try it...!

Hi, I just realized that you can also mix some small % of CaCO3 through your soil prior to the column experiment. You can test the end pH in a simple batch test. That will make the pH of your effluent even more stable.

The pH of soil will approach to 7; no matter what the sandy soil pH was due to the formation of carbon dioxide for calcaleous soil or consumption of proton by redox reaction.

Hamed,

Some years ago I carried out a similar experiment with a somewhat different objective. In the end I settled on a washed, mined silica sand (to assure no unpredictable carbon sources) and preloaded it with CaCO3 (1% by wt. as I recall) to stabilize pH. I had originally anticipated that the silica sand would be sufficiently clean and inert that I could let the incubation run and let microbial activity control the pH. I was wrong. The silica sand developed a lower than desirable pH relatively quickly after wetting. The CaCO3 addition provided an easy fix. I am not clear that CaCO3 preloading would be workable in your case since I assume it is among your target analytes. Could still be done, but would require quantitative control and measurement of preloaded CaCO3, which should be do-able if you are already set up to quantitatively measure Ca CO3 anyway. Once the bugs got going in my system, the pH stabilized at acceptable levels.

Likely someone said this, but get a sample of the sand and put it in a beaker of the solution. Check the pH. Or use a packed column and flow the solution through it.

The solution pH could change dramatically (say from pH 2 to pH 8)....or not at all.

Basically your question introduces 2 very broad variables that make it hard to address. First is the solution. What is the beginning pH? What is the ionic strength? What is the chemistry? Second is the sand. What is the sand made of (e.g. clean quartz, basaltic, decomposed granite)? What is the permeability?

or

What is the flow rate of the solution? That matters too. If the rate is slow, you may reach equilibrium with the solution. If the rate turns into a constant flow, you will reach steady state. If the flow is episodic ....etc...etc....

You might be able to model it with something like CrunchFlow, but you would need some detail about the parameters.

Sorry I am not more helpful, but I do think a lab test is in order. Just my thoughts.

Apologies - just noticed I should have read down further! You did provide more detail.