# Filtration methods at removing ions present in water

I'm doing a lab where I have to first find out what ions are present in the water (easy part). The next part is to filter out those ions from the water. Here are some methods that are stated on the paper: Coffee filter, Your own design, Britta filter. I've done my research and just can't seem to find what ions coffee filters and Brita filters filter out.

My question is: What are some own designs that i can create to filter out some ions ($\ce{Ca^{2+}}$, $\ce{Pb^{2+}}$, $\ce{Ba^{2+}}$, $\ce{Zn^{2+}}$, $\ce{SO4^{2-}}$, $\ce{PO4^{3-}}$, $\ce{Cl-}$, and $\ce{OH-}$). Or do those filters actually filter any of those ions?

• What do you need your final product to be? You can always distill to get pure water or use a dialysis bag to get the ions out. Jan 17 '17 at 6:24
• There are different kinds of Britta filters. See brita.com/why-brita/what-we-filter
– MaxW
Jan 17 '17 at 6:55
• You can't filter out $\ce{OH-}$. If you filter out one of them, more shall come to take its place.
– DHMO
Jan 17 '17 at 12:37

Cations and anions of water-soluble salts cannot be removed from water using only mechanical filters (which rely on particle size) such as coffee filters.

In order to remove such ions, ion exchange resins can be used. These resins consist of an insoluble matrix, e.g. polystyrene that is crosslinked with divinylbenzene to vary the porosity of the resin. To convert the resin into an ion exchanger, the polymer structure is functionalized with many functional groups. Typical functional groups are

• sulfonate ($\ce{R-SO3-}$) for strong acid cation exchange resins,
• quarternary ammonium (e.g. $\ce{R-N(CH3)3+}$) for strong base anion exchange resins
• tertiary amine (e.g. $\ce{R-N(CH3)2}$) for weak base anion exchange resins

The counterion of new cation exchange resins used for water treatment is usually $\ce{H+}$ or $\ce{Na+}$ (other forms can be used for special applications, e.g. $\ce{Li+}$ in pressurized water reactors), i.e. $\ce{R-SO3- H+}$ or $\ce{R-SO3- Na+}$. These counterions can be replaced by another cation from the water. Thus, a cation exchange resin can remove various cations from the water and replace them with an equivalent amount of $\ce{H+}$ and $\ce{Na+}$, respectively, e.g. $$\ce{2 (R-SO3- H+) + Ca^2+ <=> (2 R-SO3- Ca^2+) + 2 H+}$$

The counterion of new anion exchange resins is usually $\ce{OH-}$ or $\ce{Cl-}$. Thus, an anion exchange resin can remove various anions from the water and replace them with an equivalent amount of $\ce{OH-}$ and $\ce{Cl-}$, respectively.

In order to remove both kinds of ions, the water has to be passed through a cation exchanger and through an anion exchanger. Alternatively, a mixed-bed ion exchanger (which contains cation exchange resins and anion exchange resins) can be used. They are even more effective but cannot be easily regenerated.

Typical Brita water filter cartridges contain ion exchange resin beads to remove undesired ions and activated carbon to absorb other undesired compounds.

You may consider getting a hold of an ion-exchange resin of some kind. These actually replace ions in the filtered sample, e.g. $\ce{Ca^{+2}}$, with another (more innocuous) ion, e.g. $\ce{Na+}$.

I used to do a lot of research on an IMLIG (immobilized ligand) material (basically organic polymer strands stuck on silica gel) called Octolig that removed anions (phosphate, nitrate, fluoride, arsenate, etc.) and heavy metals from water.

Also look up solubility rules for your selected ions and try to find a counter-ion that will form an insoluble salt. You could treat your sample with another salt and precipitate out the unwanted ions physically. e.g. to remove $\ce{Cl-}$, you could add a lead or silver salt.

FYI a coffee filter will be essentially useless for removing aqueous ions. It's much more suitable for capturing solid particles. The pores are much larger than the ions and basically made of paper that doesn't have any substantial ionic interaction.