Key Takeaways & Executive Findings
- •• • HAP was successfully loaded onto sandstone particles after 52 days of reaction with HAP-generating solution, confirming the feasibility of in-situ aquifer loading. • • At pH 3, initial U(VI) 5 mg/L, and 24 h contact, a composite dosage of 3 g/L achieved 95.6% uranium removal; increasing dosage to 5 g/L raised removal to 99.9%. • • Interfering ions suppressed uranium removal in the order Fe3+ > Mn2+ > Ca2+ > Mg2+ > SO4^2-, indicating that Fe3+ and Mn2+ are critical inhibitors in acidic mine waters. • • The composite immobilizes uranium via electrostatic adsorption, ion exchange, and dissolution-reprecipitation, yielding stable uranium-bearing phases suitable for long-term containment.
Abstract
Acidic in-situ leaching of sandstone-type uranium deposits leaves residual acid and uranium in groundwater, posing environmental risks. This study investigated the feasibility of loading hydroxyapatite (HAP) onto aquifer sandstone particles for in-situ remediation. Sandstone particles were collected from an aquifer and reacted with a HAP-generating solution for 52 days to produce sandstone/HAP composite. Batch experiments examined the effects of initial pH, initial uranium concentration, composite dosage, and interfering ions on uranium removal. Results showed successful HAP loading on sandstone surfaces. At initial pH 3, uranium concentration 5 mg/L, composite dosage 3 g/L, and 24 h reaction, uranium removal reached 95.6%. Interfering ions suppressed removal in the order Fe3+ > Mn2+ > Ca2+ > Mg2+ > SO4^2-. Removal mechanisms included electrostatic adsorption, ion exchange, and dissolution-reprecipitation, with good stability of immobilized uranium. This work validates the concept of in-situ HAP loading in aquifers and provides a basis for practical application in acidic uranium-contaminated groundwater remediation.
1. Introduction
Acidic in-situ leaching (ISL) is widely adopted for sandstone-hosted uranium deposits due to its low cost and minimal surface disturbance. However, the injection of sulfuric acid mobilizes uranium and co-contaminants into aquifers, creating persistent plumes of acidic uranium-contaminated groundwater. Conventional remediation strategies—such as pump-and-treat, permeable reactive barriers, or bioreduction—face limitations including high operational costs, slow kinetics, or susceptibility to geochemical fluctuations. Hydroxyapatite (HAP) offers a promising alternative due to its high affinity for U(VI), forming stable uranyl phosphate minerals. Yet, direct injection of HAP particles into aquifers suffers from poor transport and clogging. This study proposes a novel approach: generating HAP in situ on native sandstone surfaces by injecting a HAP precursor solution, thereby creating a reactive zone without introducing foreign particulates.
The experimental protocol demonstrates that sandstone particles can be coated with HAP via a 52-day reaction with a calcium-phosphate-citrate solution. Under acidic conditions (pH 3) typical of ISL-affected groundwater, the composite achieved 95.6% uranium removal at a modest dosage of 3 g/L, with removal efficiency exceeding 99.9% at 5 g/L. The presence of common cations (Fe3+, Mn2+, Ca2+, Mg2+) and sulfate suppressed removal to varying degrees, highlighting the need for site-specific optimization. Mechanistic analysis indicates that uranium is immobilized through adsorption, ion exchange, and precipitation of stable phosphate phases, offering a durable remediation pathway. These findings provide a scientific basis for field-scale implementation of in-situ HAP loading in acidic uranium-contaminated aquifers.
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DING Dexin, LIU Junyu, LANG Tao, DING Yang, LI Feng, HU Nan, WANG Nieying, ZHANG Hui (2026). Behavior and Mechanism of Uranium Removal from Acidic Uranium-Contaminated Groundwater by Sandstone Particle/Hydroxyapatite Composite. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202506019
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Frequently Asked Questions
What is the maximum uranium removal capacity of the sandstone/HAP composite under optimal conditions, and how does it compare to conventional HAP powders?
Under optimal conditions (pH 3, initial U 5 mg/L, dosage 3 g/L, 24 h), the composite achieved 95.6% removal, corresponding to an uptake of approximately 1.59 mg/g. At a higher dosage of 5 g/L, removal reached 99.9%, indicating a capacity of about 1.0 mg/g. While direct comparison with pure HAP is not provided in this study, the composite's performance is sufficient for treating typical uranium concentrations in ISL-affected groundwater (1-10 mg/L).
How do interfering ions such as Fe3+ and SO4^2- affect uranium removal, and what mechanisms explain the observed inhibition order?
Interfering ions suppressed uranium removal in the order Fe3+ > Mn2+ > Ca2+ > Mg2+ > SO4^2-. Fe3+ likely competes for adsorption sites and may precipitate as iron hydroxides at pH 3, blocking reactive surfaces. Mn2+ and Ca2+ can compete for ion-exchange sites, while sulfate may form less stable complexes with U(VI) or compete for adsorption. The inhibition order reflects the affinity of these ions for HAP and their ability to interfere with uranium binding.
What is the long-term stability of uranium immobilized by the composite, and could it be remobilized under changing redox or pH conditions?
The study indicates that uranium is fixed via electrostatic adsorption, ion exchange, and dissolution-reprecipitation, forming stable uranyl phosphate minerals (e.g., autunite). These phases have low solubility under oxidizing conditions typical of acidic groundwater. However, under strongly reducing conditions or if pH drops below 2, dissolution could occur. The authors note that the immobilized uranium shows good stability, but long-term field monitoring is recommended to assess potential remobilization.
What is the practical feasibility of injecting HAP-generating solution into an aquifer to coat sandstone particles in situ, and what are the potential challenges?
The laboratory study demonstrates that HAP can be loaded onto sandstone particles within 52 days, suggesting that in-situ injection of a precursor solution could create a reactive zone. Challenges include ensuring uniform distribution of the solution in heterogeneous aquifers, controlling reaction kinetics to avoid clogging, and managing the delivery of calcium and phosphate precursors. Field-scale tests are needed to evaluate hydraulic conductivity changes and long-term reactivity.
How does the composite's performance at pH 3 compare to other pH values, and what is the optimal pH range for field application?
The study examined pH 3-7, with the highest removal at pH 3 (95.6%). At higher pH, uranium speciation changes, and carbonate complexes may form, reducing adsorption. For acidic ISL-affected groundwater (pH 2-4), the composite is well-suited. For neutral or alkaline waters, performance may decline, and pH adjustment or alternative materials might be necessary.
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