Arsenite — the As(III) oxyanion family — behaves very differently from arsenate (As(V)) in water treatment because of its acid-base chemistry. Its first dissociation constant is high, pKa₁ ≈ 9.2:
H₃AsO₃ ⇌ H₂AsO₃⁻ + H⁺ pKa₁ ≈ 9.2
f(neutral) = 1 / (1 + 10^(pH − pKa₁))
Below pH ≈ 9 arsenite is mostly the neutral molecule H₃AsO₃ — it carries no charge, so it cannot be pulled onto a nanoparticle surface by electrostatics the way arsenate (charged across almost the whole pH range) can. Instead it must bond directly to surface hydroxyl sites on an iron-oxide nanoparticle (≡FeOH) by ligand exchange, forming an inner-sphere surface complex:
≡FeOH + H₃AsO₃ → ≡FeOAs(OH)₂ + H₂O
This ligand-exchange route peaks near the nanoparticle's point of zero charge (PZC ≈ 8.5 for iron oxide), where the surface still carries enough ≡FeOH₂⁺ / ≡FeOH sites available for exchange, and falls off both at very low pH (surface saturated with H⁺, fewer exchangeable sites) and at very high pH (surface strongly negative and arsenite itself starts converting to the anion, which competes for different sites). The simulator models the resulting adsorption probability as a Gaussian centered on this optimum:
P(pH) = exp(−(pH − 8.5)² / (2·2.6²))
Each As(III) particle undergoes Brownian motion; when it drifts within capture range of a nanoparticle it is captured with probability P(pH) per contact, further reduced by the competing-anion load (phosphate and silicate occupy the same ≡FeOH sites in real groundwater treatment). Captured ions turn blue and orbit their host nanoparticle.
- pH slider — sets both the speciation split (bar chart) and the capture probability via the Gaussian above.
- Nanoparticle dose — more surface area/sites in the reactor, so ions are captured faster.
- Competing anions — models phosphate/silicate site-blocking common in real groundwater, damping P(pH).
- Reseed — refills the reactor with fresh dissolved As(III) so you can watch removal build up again.
Real-world relevance: this is the mechanism behind iron-oxide nanoparticle adsorbents used to treat arsenic-contaminated groundwater (e.g. in Bangladesh, West Bengal, parts of the US) — and the reason such systems are usually run at a mildly alkaline pH rather than acidic, unlike arsenate removal.