Below is the same Fe(OH)₃ example, already in English and slightly cleaned for direct use in your report.

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Subsurface Reactor – Example Geochemical Scenario: Fe(OH)₃ Precipitation Front

1. Basic reaction scheme

In groundwater, dissolved iron tends to precipitate under oxic and mildly alkaline conditions:

• Oxidation:
  $$\text{Fe}^{2+} + \tfrac{1}{4}\text{O}_2 + \tfrac{5}{2}\text{H}_2\text{O} \rightarrow \text{Fe(OH)}_3(s) + 2\text{H}^+$$

A simplified hydroxide precipitation step can be written as:

• Precipitation (schematic):
  $$\text{Fe}^{3+} + 3\text{OH}^- \rightarrow \text{Fe(OH)}_3(s)$$

The design goal in the Subsurface Reactor is to let a groundwater plume move towards the gallery while its pH and redox conditions are shifted in favour of Fe(OH)₃ precipitation, so that iron is immobilised before reaching the gallery.

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2. Initial groundwater composition (example)

Assume the native groundwater entering the reactor domain has:

• $$[\text{Fe}^{2+}]_0 = 1.0 \times 10^{-5} \ \text{mol/L}$$ (≈ 0.56 mg/L Fe²⁺)
• pH$$_0$$ = 6.5 (slightly acidic to neutral)
• Dissolved oxygen (DO) ≈ 2 mg/L (moderately low)
• Temperature ≈ 15 °C

Under these conditions iron is mostly present as Fe²⁺ and may be below saturation with respect to Fe(OH)₃ (Ω < 1); no significant precipitation occurs and iron remains in solution.

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3. Reactor design: pH and redox front

Within the Subsurface Reactor, a controlled alkaline and oxygen‑rich injection is applied from the gallery:

• Injection water: pH ≈ 9.0, DO ≈ 8–10 mg/L, negligible Fe.
• Injection rate $$Q_{\text{inj}}$$: chosen so that a “reaction zone” with a radius of ~10–50 m forms around the gallery.

Groundwater flows towards the gallery; Fe²⁺‑bearing water mixes with the alkaline, oxic front. In the mixing zone:

• pH rises from 6.5 to about 7.5–8.0.
• Redox potential (Eh) and DO increase, accelerating Fe²⁺ → Fe³⁺ oxidation.

Under these conditions, in the mixing zone the reaction
$$\text{Fe}^{3+} + 3\text{OH}^- \rightarrow \text{Fe(OH)}_3(s)$$
is strongly favoured:

• $$[\text{Fe}^{3+}]$$ and $$[\text{OH}^-]$$ increase.
• The ion activity product $$Q = [\text{Fe}^{3+}][\text{OH}^-]^3$$ becomes large.

Because the solubility product $$K_{\text{sp}}$$ of Fe(OH)₃ is extremely small (~10⁻³⁸), even modest increases in pH and Fe³⁺ lead to very high supersaturation (Ω ≫ 1). As a result:

• Fe precipitates as Fe(OH)₃ flocs and coatings within the reaction zone.
• Iron and co‑associated trace elements (e.g. arsenic, some trace metals) are scavenged onto this solid phase.

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4. Mass balance and simple removal efficiency

Assume the groundwater flux towards the gallery is:

• Groundwater flow rate: $$Q_{\text{gw}} = 10\ \text{L/s}$$.

Feed iron load:

• $$[\text{Fe}]_0 = 1 \times 10^{-5} \ \text{mol/L}$$.
• Molar mass of Fe ≈ 55.85 g/mol.
• Concentration ≈ 0.56 mg/L Fe.

Hourly iron input:

• 10 L/s → 36,000 L/h.
• Hourly Fe input ≈ 0.56 mg/L × 36,000 L/h ≈ 20,160 mg/h ≈ 20.2 g/h.

If 90% of Fe is precipitated as Fe(OH)₃ in the reaction zone:

• Precipitated Fe ≈ 18 g/h.
• On a yearly basis: 18 g/h × 24 h × 365 ≈ 157 kg Fe/year.

This is a modest mass on industrial scales but a clearly measurable accumulation for a scientific pilot, and it corresponds to substantial iron removal from the water reaching the gallery. The numbers can be increased by enlarging the reaction zone, adjusting concentrations or operating multiple galleries.

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5. Implications for the Subsurface Reactor concept

This example illustrates for the Subsurface Reactor:

1. Control parameters

• Injection water properties: pH, dissolved oxygen, flow rate $$Q_{\text{inj}}$$.
• Aquifer properties: hydraulic conductivity K, effective porosity $$n_e$$, gradient i → contact time.

1. Target outputs

• Fe(OH)₃ accumulation rate (kg/year).
• Reduction of Fe concentration in water at the gallery (treated groundwater quality).

1. Physical and operational constraints

• Potential pore clogging (permeability reduction due to precipitation).
• Impacts of pH/Eh changes on other minerals and on microbial communities.

In the report you can place this as:

Appendix A.3 – Illustrative Fe(OH)₃ precipitation scenario in a pilot subsurface reactor.

If you like, the next step can be a parallel carbonate‑precipitation scenario (MCO₃) written in the same style, so you have two contrasting geochemical “modes” for the reactor.