Treating Metal-Laden Groundwater at a School Well
A school well beside a former metal-plating workshop is used for irrigating a vegetable garden. Laboratory analysis reports Ni²⁺ 0.08 mg/L, Cu²⁺ 0.30 mg/L, pH 5.8, alkalinity 10 mg/L as CaCO₃, and dissolved oxygen 6 mg/L. The local irrigation guideline is 0.20 mg/L for both Ni and Cu, so only Cu exceeds. Available materials are a 20 kg bag of activated carbon, 25 kg of ground CaCO₃, 1 L of 1.0 M NaOH, and a cation-exchange column rated for 500 bed volumes before breakthrough. A vendor proposes passing the water through activated carbon and adding HCl to pH 4.5 to 'keep the metals dissolved and prevent scaling.' The chemistry club must judge the proposal, propose a safer treatment train, and say what should be measured before water is used. Ksp values: Ni(OH)₂ = 5.5×10⁻¹⁶; Cu(OH)₂ = 2.2×10⁻²⁰.
1. Evaluate the vendor's proposal using the chemistry of the metals and the guideline values. Include at least one quantitative check at pH 5.8 or at the proposed pH 4.5.
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Activated carbon is mainly useful for dissolved organic compounds; it is a poor primary treatment for ionic Cu²⁺ and Ni²⁺. Acidifying to pH 4.5 does the opposite of removal: it increases metal solubility and mobility. At pH 4.5, [OH⁻] = 10⁻⁹·⁵ = 3.16×10⁻¹⁰ M. For Cu, 0.30 mg/L = 0.30/63.55 = 4.72×10⁻⁶ M, so the ion product is (4.72×10⁻⁶)(3.16×10⁻¹⁰)² = 4.7×10⁻²⁵, far below Ksp = 2.2×10⁻²⁰. Ni is even further below its Ksp. No hydroxide precipitation is expected, so the acidified water would carry Cu through the system. At the existing pH 5.8, [OH⁻] = 6.3×10⁻⁹ M, and the Cu ion product is about 1.9×10⁻²², still undersaturated. The proposal should be rejected: it does not remove Cu and may increase the amount reaching the garden.
2. Design a treatment train that would bring Cu below 0.20 mg/L and keep Ni below its guideline. State the pH target, the role of each unit, and a monitoring plan.
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Raise pH to about 8.5–9.0 with ground CaCO₃, using NaOH only for fine adjustment. At pH 9.0, [OH⁻] = 1.0×10⁻⁵ M, so the Cu ion product is (4.72×10⁻⁶)(1.0×10⁻⁵)² = 4.7×10⁻¹⁶, vastly above Ksp = 2.2×10⁻²⁰; Cu(OH)₂ should precipitate. For Ni, (1.36×10⁻⁶)(1.0×10⁻⁵)² = 1.36×10⁻¹⁶, slightly below Ksp = 5.5×10⁻¹⁶, so Ni is not fully removed but is already below its 0.20 mg/L guideline. Let the precipitate settle or filter it, then polish with the cation-exchange column to capture residual Ni and Cu. The column's 500-bed-volume rating is finite, so track throughput and regenerate or replace it before breakthrough. Monitor pH, Cu, Ni, alkalinity, and turbidity after each batch; do not irrigate until Cu is below 0.20 mg/L and pH is in an agronomically acceptable range. Dispose of metal-bearing sludge as hazardous or controlled waste, not on the garden.