Science Buddies: "Ask an Expert"

Hi. I'm doing an experiment to find how bad my highschools water is. I looked up the different ions that you usually find in bad pipes and decided to focus on iron 2/3 and zinc. I researched and found indicators for all 3 and then got a sample of the water and tested it for the ions. I then took distilled water and added amounts of the metals to set up an absorbance scale using a colorimeter in class and found the concentration of metals in the water.

I am pretty much finished my original experiment, but I am now trying to find out how to treat the water. I'm more interested in using natural materials like sand or dirt, but could do a Brita filter (charcoal) if I had too. My question is on the math behind the filtering. I can't find anything on what sort of cleaning of the water I should see. Any help would be appreciated.

If you really need pure water, reverse osmosis is the way to go. Use it as a search term. It is more energy-efficient than distillation. You may be able to find low-cost reverse osmosis devices that operate on household water pressure.

You might want to rethink "bad" in terms of iron and zinc. Many people take mineral supplients that include iron and zinc as humans need these in their diet.

The older filtering methods commonly referred to as water softners or iron filters involve a resin bed that is recharged using sodium cloride (or potasium cloride for those trying to reduce their sodium intake) or potassium permanganate (iron filters). In these applications, the resin beads act as a positive ion exchange where sodium or potassium bound to the resin is freed and replaced by iron, zinc, calcium, etc.

Activated charcoal filters work primarily on negative ions and remove clorine and floride. To prevent bacterial growth, a charcoal filter should only be used on clorinated water supplies and should be the last filter in any sequential filter setup.

Sand, paper, and Diatomaceous Earth filters are commonly used on pool systems because they don't filter out minerals very well. Water in Marcite pools MUST be maintained at controlled total hardness to prevent excess marcite loss so the choice of these kinds of filters is manditory.

A previous responder indicated that reverse osmosis filtration is a better modern method; however, it is only suitable for low flow rates. Again, it will take out chlorine and florides and like charcoal filters must only be used with clorinated water to prevent bacterial growth.

This may not be practical as an engineering solution, but Fe is typically very insoluble in aerobic water with neutral pH. If you have substantial concentrations of iron in solution, you can oxidize the water and/or adjust pH to precipitate out the Fe. I'm happy to discuss more if you're interested.

I have researched water softening and it seems to me that it does not have much to do with my experiment as it focusing on alkinity of water. I have decided to focus on removing zinc (the only ion in large concentrations that i found) through filtration through peatmoss. My design does not have to be realistically applicable and I am just looking for what mathematical formulae will predict the amount of cleaning I can expect through (I believe the correct term is) adsorbance. Thank you all for your help.

I would suggest you try using Google to get information about this specific issue. Search terms that worked well for me were "adsorbent peat moss zinc", which resulted in numerous hits pointing towards scholarly documents.

The most useful looking one to me "Equilibrium Parameters for the Sorption of Copper, Cadmium and Zinc Ions onto Peat", unfortunately, was only available for purchase from Wiley Interscience. However, it might be available through your local library for free, especially if you have access to any nearby university libraries. If it looks useful to you, based on the abstract, I would suggest you get the citation information from that link on the Google results and take it to a reference librarian and ask for help.

Also, the Wikipedia article on Adsorption appears to have considerable discussion on the math of the process. I wouldn't use that as a primary research source, because it's just a (potentially unreliable) encyclopedia, but it may be able to direct your research towards more traditional sources you could cite for your project.

Hope that's helpful.

Just to make a little correction to one of the above answers. Chlorine destroys Reverse Osmosis membranes. Therefore you'll always see a carbon filter that removes chlorine (or chloramine, depending on your local water supply) before it goes through the membrane.

A further filtering step is often the addition of mixed-bed anion/cation resins after the RO step. This is known as a deionization system (or DI for short). This produces water with virtually no ions in it. Not totally safe for drinking for esoteric reasons of increased reactivity, but as low in dissolved solids (pure) as can be easily obtained (outside of distillation, or other methods.)

You can easily find inexpensive dissolved solids meters on the web. These can give you a numeric reading of dissolved solids, but won't tell you what they are. They can read a very high number for hard water, and a low number for soft water, lower still for RO water, and essentially 0 for DI water.

As has been noted above, all these filtration systems are only good for low flow situations, unless you have a storage tank (then without something like a UV light, bacterial contamination becomes an issue.)