Arsenic Remediation

Where arsenic comes from and how it behaves

Arsenic is a naturally occurring metalloid, and at many sites the groundwater problem is geologic: iron oxides in the aquifer hold arsenic until reducing conditions dissolve them and release it. Human sources add to that background, including mining and smelting, chromated copper arsenate (CCA) wood treatment, lead arsenate pesticides on former orchards, glass manufacturing, coal ash and pesticide formulation. Inorganic arsenic compounds are classified as human carcinogens, and the federal drinking water standard is 10 µg/L.

In water arsenic occurs in two oxidation states, and the difference matters for treatment. Arsenate, As(V), is the form in oxygenated water. It carries a negative charge at environmental pH (H₂AsO₄⁻ and HAsO₄²⁻) and binds strongly to iron oxide surfaces. Arsenite, As(III), is the form in reducing water. It is uncharged below about pH 9 (H₃AsO₃), more toxic, more mobile, and held less firmly by iron oxides at neutral pH. Nearly every arsenic remedy therefore does two things: oxidizes arsenite to arsenate where needed, and supplies an iron surface for the arsenate to bind to.

How zero valent iron treats arsenic

Ferox zero valent iron treats arsenic by corroding. As the metallic iron oxidizes in water it continuously produces fresh iron oxyhydroxides, the same ferrihydrite, lepidocrocite and magnetite phases that hold arsenic in natural aquifers, and arsenate and arsenite adsorb onto and co-precipitate with those corrosion products. Under strongly reducing conditions a portion of the arsenic is reduced and incorporated into the iron mineral structure or, where sulfate is being reduced, into arsenic sulfides. Because the iron keeps corroding, the sorbent is renewed in place rather than exhausted in one pass, which is what sets zero valent iron apart from a fixed-bed iron oxide sorbent.

There is no simple stoichiometry for arsenic sorption. Capacity depends on pH, on how much arsenite versus arsenate is present, and on competing anions, particularly phosphate and silicate, which bind to the same surface sites. Column or batch tests on the actual water set the iron loading and the contact time; Hepure provides 1-lb and 5-lb samples for that work.

Where the iron goes depends on the problem. Granular Ferox PRB is the media for a permeable reactive barrier across an arsenic plume and for above-ground reactors or filter beds, with 96% of particles in the 20/60 mesh range and no rust, dust or oil. The finer Ferox Flow and Ferox Target powders are injected as a slurry to build a treatment zone in place. Our zero valent iron remediation guide covers how particle size and delivery method are chosen.

Iron coagulation and permanganate pre-oxidation

For water in a tank, whether a treatment system, a dewatering stream or mine drainage, the established arsenic remedy is coagulation with an iron salt followed by filtration. Ferrous sulfate is the iron source: once oxidized to ferric iron, it forms hydrous ferric oxide floc that adsorbs arsenate and settles or filters out. Iron is dosed in large excess over the arsenic, and the ratio is set by jar testing on the actual water, because alkalinity, phosphate and silicate all change it. Arsenite is removed poorly by this route, so water with significant As(III) is pre-oxidized first.

Potassium permanganate is a standard pre-oxidant for arsenite. The reaction 3 H₃AsO₃ + 2 MnO₄⁻ → 3 HAsO₄²⁻ + 2 MnO₂ + H₂O + 4 H⁺ works out to about 1.4 lb of KMnO₄ per pound of arsenite before any other oxidant demand in the water, and the manganese dioxide it produces is itself a sorbent that is removed with the iron floc. Sodium permanganate gives the same chemistry as a liquid feed. Where the pH needs raising for the floc to form, sodium hydroxide is supplied as a 25% solution.

Sulfide precipitation with Remotox

Remotox calcium polysulfide precipitates arsenic as arsenic sulfide alongside the lead, cadmium and copper it is more often used for. The limitation is pH: arsenic sulfides are insoluble below about pH 7 and redissolve above it, so Remotox is the right choice for arsenic only where the treated water or soil will stay on the acidic side, or where arsenic is a secondary contaminant at a chromium or lead site that is being treated with polysulfide anyway.

What decides success at an arsenic site

Speciation. Measure arsenite and arsenate separately. A water that is mostly As(III) needs an oxidation step, or a reductive approach that relies on iron corrosion products, not a plain ferric coagulation.

Competing anions. Phosphate and silicate compete with arsenate for iron surface sites and raise the iron dose. High-phosphate water is the usual reason an arsenic system underperforms its bench test.

Redox after treatment. Arsenic held on iron oxides is released if those oxides are later dissolved by reducing conditions. That is why adding an organic electron donor for chlorinated solvent bioremediation can mobilize arsenic in the same aquifer, and why zero valent iron, which keeps adding iron, is the amendment of choice where arsenic and solvents overlap.

pH. Arsenate sorption on iron oxides is strongest in acidic to neutral water and falls off above about pH 8. Where the water is alkaline, the dose goes up or the pH comes down.

What Hepure provides

We supply the chemistry and help you size it: Ferox zero valent iron from 1-lb and 5-lb samples through 100-lb bags, 1,000-lb barrels and 2,205-lb super sacks; ferrous sulfate in 50-lb bags with a certificate of analysis; potassium permanganate in 55-lb pails and 330-lb drums or as a pre-mixed solution; sodium permanganate and sodium hydroxide in drums and totes; and Remotox in pails, drums, totes and tankers. Send Pat or Bob the arsenic concentration and speciation if you have it, the pH, the flow or volume, and whether the work is a barrier, an injection or a tank system, and you will get a product recommendation and a quantity estimate. Lab treatability testing and column studies for barrier design are available. We do not design the remedy or operate the system; that stays with your engineer.

Common questions

Does zero valent iron remove arsenite as well as arsenate? Yes. Iron corrosion products sorb both, and the reducing conditions at the iron surface do not depend on prior oxidation. Capacity for arsenite is lower at neutral pH, which the bench test accounts for.

How long does an iron barrier last for arsenic? Until the reactive iron is consumed or the pore space fills with precipitate, typically years. Ferox Flow and Ferox Target carry a rated longevity of 3 to 15 years for injected applications; barrier life is set by the groundwater chemistry and is estimated from column tests.

Can the arsenic come back? Only if the iron oxides holding it are dissolved by a later change to strongly reducing conditions, or desorbed by a large rise in pH or phosphate. Monitoring redox and pH downgradient is the guard against both.

Is a bench test necessary? For arsenic, always. Sorption capacity varies more from water to water than for almost any other contaminant, and the competing-anion effect cannot be estimated from a concentration alone.