Reverse osmosis removes heavy metals from drinking water with 97–99.9% efficiency, making it one of the most effective filtration methods available for home use. RO systems can filter out lead, arsenic, cadmium, chromium, mercury, and other dissolved metals that carbon filters alone typically miss.
How RO Membranes Block Heavy Metals
RO membranes work through a combination of physical and electrical barriers. The membrane contains pores small enough to filter particles down to 0.0001 micrometers, which is far smaller than dissolved metal ions. When water is forced through under pressure, three mechanisms determine what gets through and what doesn’t.
First, there’s a size effect: ions have to squeeze through pores that may be too narrow for them, especially once you account for the shell of water molecules that clings to each ion. Second, the membrane carries an electrical charge that repels ions of the same charge, pushing them away from the surface. Third, the energy required for a charged particle to move from water into the membrane material creates an additional barrier. Together, these three forces give RO membranes their remarkably high rejection rates for dissolved metals.
Removal Rates for Specific Metals
Not all heavy metals are removed equally. Most are filtered at rates above 95%, but a few present challenges depending on their chemical form.
- Lead: RO systems certified under the NSF/ANSI 58 standard must reduce lead from 150 parts per billion down to 10 ppb or less. Most systems exceed this requirement.
- Cadmium: Lab testing shows removal rates around 98% across a pH range of 4 to 7.
- Chromium: Both hexavalent and trivalent chromium are included in NSF/ANSI 58 optional testing claims, and RO handles both forms effectively.
- Arsenic: This is where things get complicated. Arsenic exists in two common forms in water. The pentavalent form (arsenate) is removed at over 99% from pH 6 to 11. The trivalent form (arsenite) is far harder to catch, with removal rates of only 40–60% under normal conditions.
- Copper, barium, selenium, radium: All are included in the NSF/ANSI 58 certification framework, with typical removal above 95%.
The Arsenic Problem
Arsenic deserves its own discussion because the difference between its two forms is dramatic. Pentavalent arsenic carries a stronger electrical charge in water, which makes it easy for the membrane to repel. Trivalent arsenic, by contrast, is uncharged at typical drinking water pH levels, so it slips through more easily. At pH 6, a standard RO membrane catches only about 40% of trivalent arsenic. Raising the pH to nearly 11 pushes removal up to around 97%, but that’s not practical for most home systems.
The better approach is oxidation: converting the trivalent form into the pentavalent form before the water reaches the membrane. Research at Marquette University demonstrated that adding a simple electrochemical pretreatment step pushed total arsenic removal above 90% even when the starting water contained the harder-to-remove form. If you live in an area with known arsenic contamination, it’s worth checking which form is dominant in your water supply. A system designed only for standard RO filtration may leave a meaningful amount of trivalent arsenic behind.
What Affects Performance Over Time
An RO membrane doesn’t perform identically on day one and day 500. The two biggest factors that degrade performance are fouling and feed water chemistry.
Fouling happens when particles, mineral scale, or biological growth accumulate on the membrane surface. EPA testing found that water flow through the membrane dropped 20–40% over several hundred hours of operation when iron oxide deposits built up on the surface. In one test, however, a membrane maintained 99.8% copper rejection over 550 hours despite the flux decline, suggesting that even a partially fouled membrane can still reject metals effectively for a while. The concern is that eventually, reduced flow forces the system to work harder or pass more contaminants.
Feed water pH also matters. The membrane’s electrical charge changes depending on how acidic or alkaline the incoming water is, and that charge is a key part of how it rejects dissolved metals. Systems operating outside their designed pH range may see lower rejection rates. Most residential RO systems are designed for municipal tap water in the pH 6.5–8.5 range, so this is mainly a concern if you’re on well water with unusual chemistry.
What to Look for in Certification
The NSF/ANSI 58 standard is the benchmark for residential RO systems in the United States. Every certified unit must demonstrate total dissolved solids reduction. Beyond that, manufacturers can optionally certify for specific contaminants including lead, arsenic, cadmium, chromium (both forms), barium, copper, selenium, radium, fluoride, and nitrate.
The key word is “optional.” A system certified under NSF/ANSI 58 for TDS reduction alone has not been independently verified for any specific heavy metal. If your goal is removing a particular contaminant, check the product’s certification listing for that specific claim. NSF maintains a searchable database of certified products that shows exactly which contaminants each system has been tested against.
The Mineral Tradeoff
RO membranes don’t distinguish between harmful metals and beneficial minerals. The same process that strips out lead and cadmium also removes about 97% of calcium, 96% of magnesium, 95% of iron, and 95% of zinc. The resulting water is essentially an extreme form of soft water, with calcium levels typically dropping below 6 mg/L.
For most people who get adequate minerals from food, this isn’t a health concern. But if your diet is marginal or you rely on your water as a significant mineral source, it’s worth considering remineralization. The simplest options are adding a mineral filter cartridge after the RO stage or using an alkaline water pitcher. Some systems come with a built-in remineralization stage that passes the purified water over calcium carbonate before it reaches your glass. You can also add trace mineral drops directly to your water, which gives you control over exactly what goes back in.