Arsenic & Fluoride

Iron oxide binds arsenic, not fluoride. Arsenate has a strong specific affinity for FeOOH surfaces; fluoride's affinity is for aluminum phases. The two contaminants get different media, plumbed inline ahead of the disinfection barriers, and their economics are wildly different:

Well arsenic is measured in µg/L, so a granular ferric hydroxide (FeOOH) bed treating 250→10 µg/L at 20 L/day consumes only ~1.8 kg of media a family-year (~$11). The chlorinator sharpens it: pre-oxidizing As(III)→As(V) markedly improves capture. Spent media is hazardous — stabilized in concrete or swapped as a return-to-supplier cartridge.

ECAR. Iron-based arsenic removal has a two-decade, two-continent track record under Ashok Gadgil's group at UC Berkeley and Lawrence Berkeley National Lab. Their ElectroChemical Arsenic Remediation replaces the imported sorbent cartridge with a plate of ordinary mild steel: a small current dissolves the iron in place, and the freshly formed iron-hydroxide flocs bind arsenic and settle out — the consumable becomes scrap-grade steel and a few watt-hours per liter, with no media supply chain at all. In West Bengal, India, a 10,000 L/day ECAR plant commissioned at Dhapdhapi High School near Kolkata has supplied arsenic-safe water (roughly 250 µg/L in, under 10 and typically under 5 out) to a school community of ~3,000 since 2016 — and, handed to a local operator in 2017, covers its costs selling surplus safe water to the surrounding village at well under a US cent per liter. In Allensworth, California — a historically Black-founded Tulare County town on arsenic-contaminated groundwater — the same technology came home: a 2019 farm field trial grew into an EPA-funded small-community system with remote monitoring, holding output consistently below the WHO 10 µg/L guideline, with current work on zero-liquid-discharge operation. Gadgil received the U.S. National Medal of Technology and Innovation in 2023 for this body of work — which also includes UV Waterworks and the Berkeley-Darfur Stove, the two direct ancestors of this unit's disinfection and cooking cores. For SolarHearth, ECAR is the natural cluster-scale arsenic bay where wells run hot: steel plates and a trickle of DC from the same 48 V bus, with the household FeOOH cartridge as the entry-level option.

At Rift Valley concentrations (8→1.5 mg/L) the same 20 L/day consumes 24–32 kg of media a year — a mass problem, not a chemistry problem. Activated alumina (~$95/yr, regenerable with NaOH then alum) is the standard; bone char (~$19/yr) can be made on-site by charring bones in a retort pot on the induction hob at 400–500 °C — media without a supply chain.

A note on bone char. It is not an improvisation: bone char is the longest-proven defluoridation medium in East Africa — the Catholic Diocese of Nakuru has run bone-char defluoridation in Kenya's Rift Valley for decades, exactly the fluoride belt these camps sit in. Char quality matters (under-charred bone tastes and smells; over-charred loses capacity — the retort recipe is temperature-controlled by the induction hob itself, one advantage over open-fire charring). And acceptability is a per-community question, not a footnote: bone origin carries religious and cultural weight — communities that keep halal practice may accept only verified cattle or goat bone, and some households will refuse bone-derived media entirely. The deployment rule: offer bone char where the community accepts it after consultation, default to activated alumina where it does not, and never make bone char the only option.