Reverse osmosis (RO) is a highly effective water purification technology that addresses a wide range of contaminants, including heavy metals like lead, arsenic, and mercury. These metals are naturally occurring but become toxic when concentrated in water sources through industrial activity or plumbing corrosion. RO is exceptionally effective at removing these dissolved solids, consistently achieving high rejection rates. RO targets elements that are often tasteless, odorless, and invisible in drinking water.
The Mechanism of Reverse Osmosis Filtration
Reverse osmosis operates by applying external pressure to contaminated water, forcing it through a specialized semi-permeable membrane. This membrane functions as an extremely fine physical barrier, allowing water molecules to pass while blocking nearly all dissolved solids. The pressure must overcome the natural osmotic pressure that would otherwise cause water to flow from the pure side to the concentrated side.
The effectiveness of this system relates directly to the microscopic size of the membrane’s pores (about 0.0001 micrometers). This size is significantly smaller than most ionic contaminants, including charged metal particles. As pressurized water flows across the membrane, the pure water (permeate) passes through, while rejected concentrated impurities are flushed away in a separate waste stream. This physical separation enables the high rejection rate of heavy metal ions.
The RO membrane is a surface separation process, not a depth filtration system that traps particles. Contaminants are rejected primarily based on their size and electrical charge, a principle called size exclusion and charge repulsion. Heavy metal ions carry a positive electrical charge and are generally larger than water molecules, so they are effectively repelled by the membrane material. This makes RO superior to standard physical filters for removing dissolved inorganic contaminants.
Removal Rates for Specific Heavy Metals
The high efficiency of reverse osmosis is demonstrated by the specific rejection percentages achieved for common heavy metal contaminants. For lead, which often enters drinking water from corroded household plumbing, RO systems typically achieve removal rates of up to 99%. This high performance is due to lead existing as a relatively large, charged ion in solution, making it an ideal candidate for membrane rejection.
Cadmium is routinely removed at efficiencies ranging from 90% to 98%. Mercury also sees rejection rates of up to 95%. These consistent high percentages highlight the reliability of the RO process for purifying water contaminated by multiple sources.
The removal of arsenic requires a nuanced understanding because it exists in two primary forms: pentavalent (Arsenic V) and trivalent (Arsenic III). Arsenic V is a large, charged ion and is highly rejected by the RO membrane, often exceeding 95% removal. Arsenic III is generally smaller and less charged, resulting in lower rejection rates, sometimes falling below 85%. Therefore, many effective RO systems incorporate pre-treatment, such as oxidation, to convert Arsenic III into the highly-removable Arsenic V.
Understanding the Toxicity of Heavy Metals in Water
The necessity for high-efficiency removal methods like RO is rooted in the public health threat posed by heavy metal exposure. Unlike organic contaminants that the body can metabolize, heavy metals are non-biodegradable and tend to accumulate in biological tissues over time (bioaccumulation). This accumulation leads to systemic damage because the body struggles to excrete these substances.
Lead is particularly harmful to the developing nervous systems of children, contributing to learning difficulties and behavioral problems. In adults, chronic exposure can damage the kidneys, liver, and brain. Mercury primarily targets the central nervous system and impairs kidney function.
Arsenic is a known human carcinogen, with long-term exposure linked to cancers of the skin, bladder, and lungs. It can also contribute to cardiovascular disease and neurological disorders. Cadmium exposure results in toxicity to the kidneys and liver, and weakens bone structure. Preventing the ingestion of these elements through purified water is foundational to mitigating these severe health risks.