Overview
Ion rejection refers to the selective blocking of ions by membranes or barriers based on charge, size, or affinity. This phenomenon is fundamental to technologies like reverse osmosis (RO), nanofiltration (NF), and electrodialysis. Industrial systems leverage ion rejection to purify water, recover metals, and separate chemical compounds. Modern ion rejection membranes are engineered from polymers (e.g., polyamide, cellulose acetate) or inorganic materials (e.g., ceramics). Their performance depends on factors like pore structure, surface charge, and operational conditions such as pressure and temperature.
Physical and Chemical Properties
Ion rejection efficiency is governed by membrane properties like pore diameter (typically 0.1–10 nm for NF/RO) and zeta potential (surface charge). High-performance membranes exhibit >95% rejection rates for monovalent ions (e.g., Na⁺, Cl⁻) and up to 99% for divalent ions (e.g., Ca²⁺, SO₄²⁻). Chemical stability is critical—most commercial membranes tolerate pH 2–11 but degrade under strong oxidants. Temperature limits range from 40°C (polymeric) to 80°C (ceramic). Flux rates vary from 10–100 L/m²·h depending on applied pressure (commonly 5–60 bar for RO).
Main Applications
In desalination, ion rejection membranes produce potable water from seawater (rejecting >99% salts). Municipal wastewater plants use them to remove heavy metals (e.g., Pb²⁺, Cd²⁺) and nutrients (e.g., NO₃⁻). The food industry applies ion rejection for dairy demineralization and juice clarification, while pharmaceuticals rely on it for buffer exchange and API purification. Emerging applications include lithium extraction from brines and acid recovery in metal plating.
Safety and Storage
Membrane handling requires precautions against mechanical damage (e.g., folding, scratching) and chemical exposure. Chlorine degrades polyamide membranes; sodium bisulfite is often used for dechlorination. Wet-stored membranes need biocides to prevent microbial growth. Industrial systems require pressure relief valves and conductivity monitors to detect membrane breaches. Spent membranes are typically incinerated or landfilled, with some materials (e.g., PVDF) requiring special disposal due to fluoropolymer content.
B2B Procurement Guide
Specify rejection targets (e.g., "90% Mg²⁺ removal") and feedwater composition (TDS, pH, organics). For large-scale projects, request pilot testing with actual process streams. Compare membranes by lifespan (typically 3–7 years) and cleaning protocols. Leading suppliers include DuPont (FilmTec), Toray, and Hydranautics. Bulk orders (e.g., 8040 spiral-wound elements) commonly cost $300–$800 per unit. Consider modular systems for scalability and energy recovery devices (e.g., ERDs) to reduce operational costs.
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