Reverse Osmosis for Arsenic and Uranium: Why the Form of the Contaminant Matters

If a water test comes back showing arsenic or uranium, reverse osmosis is one of the few home treatment methods generally described as effective against either one, but the honest answer to "does RO remove arsenic" depends on a detail most buying guides skip entirely: which chemical form of arsenic is actually in the water. Arsenic occurs naturally as two distinct species that behave very differently inside a membrane, so the same question can have two different answers depending on the well or municipal supply in question. Uranium is a separate story again, with its own regulatory limit and its own treatment trade-offs. This article walks through what separates the two contaminants, why a laboratory test has to come before any equipment decision, and where the Hydrion Purity RO600's own specifications become relevant once that test is in hand.

• Arsenic in groundwater is almost always naturally occurring, and it shows up as arsenite, As(III), or arsenate, As(V), depending largely on how oxygenated the aquifer is.
• Reverse osmosis membranes reject electrically charged, larger dissolved species far better than small neutral ones, which is why the two arsenic forms are removed at very different rates.
• A standard "total arsenic" lab result does not say which form is present, so a water utility facing poor rejection typically adds an oxidation step before the membrane rather than replacing it.
• Uranium is also naturally occurring, carries its own EPA limit, and is generally removed well by reverse osmosis, though the process sends a meaningful share of the feed water to drain.
• Any arsenic or uranium reduction figure on a product page is the seller's own claim rather than an independently verified test result, which matters when comparing systems.

Why Arsenic Shows Up in Two Different Chemical Forms

Arsenic that turns up in a water test is almost never industrial contamination. It leaches naturally out of the rock and sediment a well sits in, and which chemical form it takes depends largely on how much oxygen reaches that groundwater. A drinking-water fact sheet from the Oregon Health Authority explains that shallower, more oxygenated aquifers tend to hold arsenic as arsenate, As(V), while deeper groundwater with little oxygen tends to hold it as arsenite, As(III) (source: Oregon Health Authority drinking water fact sheet).

That distinction is not just a chemistry curiosity. At the pH range found in typical drinking water, the same fact sheet notes that As(V) species carry a negative electrical charge while the dominant As(III) species is close to electrically neutral. A standard lab report that lists "total arsenic" in micrograms per litre does not tell you which of the two forms you're dealing with, even though, as the next section explains, that difference decides how well a reverse osmosis membrane can remove it. Both regulators treat total arsenic the same way regardless of form: the World Health Organization's current guideline value is 10 micrograms per litre, described as provisional because of how difficult arsenic is to remove from water in practice, and the EPA's enforceable maximum contaminant level in the United States is set at the equivalent 0.010 mg/L (sources: WHO arsenic fact sheet; EPA National Primary Drinking Water Regulations).

Why the Form of Arsenic Changes What a Membrane Can Reject

Reverse osmosis membranes work by rejecting dissolved species that are too large or too electrically charged to pass through alongside water molecules. A negatively charged ion like As(V) is rejected well for exactly that reason, while a near-neutral molecule like As(III) slips through far more easily because the membrane has little electrical driving force to stop it. The Oregon Health Authority fact sheet puts a number on that gap: under favourable conditions, reverse osmosis can remove up to 95% of As(V), while As(III) removal is consistently lower because the dominant species stays close to uncharged (source: Oregon Health Authority drinking water fact sheet). That 95% figure describes a best-case laboratory condition rather than a guarantee for any specific home and water chemistry, so it's worth reading as a ceiling, not a promise.

A real-world case makes the same point from the utility side. A case study published by the American Membrane Technology Association describes El Paso Water Utilities finding that standard high-rejection RO membranes removed arsenic well at most of its brackish wellhead sites, but rejection fell below 40% at one site where the arsenic was present mainly as As(III). The utility's fix was not a different membrane; it was a full-scale prechlorination and dechlorination process that oxidized the arsenic to As(V) before the water ever reached the membranes (source: AMTA case study on arsenic oxidation prior to RO). That is also why a flat "does reverse osmosis remove arsenic" question can't have one universal answer: it depends on which form is dissolved in the water being treated, and whether anything oxidizes it first.

Why a Lab Test Comes Before Any Equipment Decision

Given how much the arsenic form changes the outcome, the first move after a concerning result is a certified laboratory test for total arsenic, with a retest if the number looks unusual, rather than a purchase. Where speciation testing is available, learning whether the arsenic present is mainly As(III) or As(V) tells you directly whether an oxidation pretreatment step is likely to matter before a membrane system goes in. The Oregon fact sheet makes the same recommendation from the regulator's side: test at a state-certified laboratory and talk to a water-treatment specialist before settling on a method, rather than choosing equipment first and hoping it fits the water.

It's worth testing for iron and manganese in the same round, since both can interfere with arsenic removal systems and change which pretreatment, if any, makes sense. A consumer TDS meter is no substitute here: it reads total dissolved solids as a rough overall number and has no way to detect arsenic specifically, let alone which chemical form it's in, so a low or normal TDS reading says nothing about arsenic risk one way or the other. Our own explainer on what TDS actually measures goes into why that number is useful for some things and not others.

Uranium: A Different Contaminant, Handled Differently

Uranium is a separate issue from arsenic, and it's worth treating that way rather than lumping the two together under "natural contaminants." It too is almost always naturally occurring in well water, coming from the gradual erosion of local rock and soil rather than from any identifiable pollution source, according to an Ohio State University Extension fact sheet on uranium in well water (source: OSU Extension / Ohio Watershed Network, uranium in well water). The EPA's enforceable drinking-water limit for uranium is 30 micrograms per litre (source: EPA National Primary Drinking Water Regulations), and the same Extension fact sheet notes that a result above that level is treated as an action item: it recommends pursuing treatment, since long-term exposure above the limit has been linked to increased cancer risk in the surrounding research. A detectable but below-limit result, by contrast, makes treatment optional rather than urgent, which is a useful distinction to keep in mind instead of treating any trace of uranium as a crisis.

Unlike arsenic, uranium's electrical behavior in water doesn't split into two problematic forms the way arsenic does, and the same Extension source lists reverse osmosis as an effective uranium treatment. The trade-off it points out is on the water-use side rather than the chemistry side: RO for uranium removal is described as comparatively slow and as sending several gallons to drain for every gallon of treated water produced. A conventional water softener is not listed as a uranium-removal method at all, since softening exchanges hardness minerals like calcium and magnesium for sodium and isn't built to address a dissolved radionuclide like uranium.

Reading the RO600's Specifications Against These Two Contaminants

Once a lab result is in hand, it's reasonable to start comparing equipment, and this is where the Hydrion Purity RO600's own specifications become relevant. Its 0.0001-micron membrane and three-cartridge stack (a PPC sediment-and-carbon stage, the 600G RO membrane, and a TC post-filter) are specified on the product's own page, and any arsenic, uranium or other dissolved-solids reduction figures stated there are the seller's own claims rather than an independently inspected test certificate, so they're worth reading with that distinction in mind rather than as lab-verified fact.

The RO600's 2:1 pure-to-drain ratio is the practical cost of the same rejection mechanism described earlier in this article: a membrane built to reject charged, dissolved species well also sends a portion of the feed water to drain to flush out what it rejects, which lines up with the same trade-off the Extension source describes for uranium treatment generally. The system is specified for cold, treated municipal water, and it isn't presented here as a stand-alone fix for an untreated well supply carrying an unconfirmed arsenic or uranium result; a well with its own pretreatment needs is a separate conversation covered in our piece on reverse osmosis for well water. None of the specification detail above changes the sequence this article has been building toward: a lab test, and ideally a speciated one for arsenic, still has to come first.

What To Do With a Confirmed Result

The path through this is more straightforward than the chemistry might suggest: test first, identify the arsenic form where speciation testing is available, treat the specific problem the test actually shows (which may mean an oxidation step ahead of a membrane, not just the membrane itself), and only then compare equipment against the result. Readers who want the broader mechanics of how reverse osmosis works before going further can start with our complete guide to reverse osmosis, and anyone weighing what else dissolved minerals in RO water do or don't include can see our article on what reverse osmosis removes from water.

Once a test confirms a treated-mains supply with a dissolved-contaminant profile that reverse osmosis is well suited to, the RO600 is one system worth comparing specification for specification against whatever else is on the shortlist. The equipment choice follows the test. It doesn't replace it.