Impact of hydrogen peroxide treatment on the binding and mobilization of uranium in soils

Hydrogen peroxide has been recently used to oxidize organic contaminants in soil as an in-situ environmental restoration technique by the Department of Energy (DOE). However, many organic-contaminated soils at DOE facilities also contain uranium (U) which may become mobile during oxidation of target organics thereby increasing the potential for groundwater contamination and human exposure. This research addressed the impact of hydrogen peroxide (H₂O₂) treatment on U binding and mobilization in soils.

Three different types of clay (Kaolinite, Illite and Ca-Montmorillonite), two metal oxides (Al₂0₃ and Fe₂O₃) and three DOE natural soils (Hanford (I), (II), and Portsmouth) were used in the research. All soils and soil materials were spiked with U which ranged from 0.157 to 5.98 mg U/g and treated with 0 to 30% H₂O₂ (v/v) for varying contact times (1-hour and 24-hour). During treatment, solution pH was monitored to assess its impact on U mobilization. In addition, soils and soil materials were pretreated with 30% H₂O₂ before U loading to evaluate the impact of peroxide on soil binding capacity.

Equilibrium binding to untreated soils, in general, increased with increasing solution pH. Based on the distribution coefficient (K_d), binding affinity increased in the order of Hanford (I) < Hanford (II) < Al oxide < Fe oxide < Portsmouth ≈ Ca-Montmorillonite < Illite < Kaolinite.

Binding affinity of U to pretreated soils was in the order of Kaolinite ≈ Illite ≈ Al oxide ≈ Hanford (I) & II << Ca-Montmorillonite ≈ Portsmouth << Fe oxide. Peroxide pretreated soils were capable of sorbing U, however, binding intensity varied with soil composition. The sorption capacity of U after H₂O₂ treatment increased for the sandy soils (Hanford I & II) and Fe oxide whereas it decreased for clays, Al oxide and the clay- like soil (Portsmouth). Enhanced U binding in Hanford soils may be due to an increase in sorption sites which resulted from stripping trace metals from the soil matrix.

Oxidation of organic carbon, dissolution of soil calcite, complexation of U with carbonate and precipitation of uranyl peroxide controlled U mobilization. Complexation and precipitation affected U mobility upon H₂O₂ treatment in different pH domains. Carbonate complexation favored U mobilization (pH > 5) whereas precipitation of uranyl peroxide complexes limited mobilization ( 2 < pH < 5).

Minimum U mobilization (< 6%) was observed in clays (Kaolinite and Ca-Montmorillonite), oxides and Portsmouth soil at H₂O₂ loadings ≥ 18% after three one-hour treatments due to precipitation of uranyl peroxide. Maximum U mobilization, however, was detected for Illite and Hanford soils during this treatment period. Enhanced mobilization from Illite was due to the oxidation of organic carbon associated with this clay whereas carbonate complexation was responsible for the increased U mobilization observed in Hanford soils.

Longer contact time (> 1) increased U mobilization from Ca-Montmorillonite and Illite, and decreased U mobilization from Hanford soils and had little impact on U mobilization from Kaolinite, oxides and Portsmouth soil. In general, most of the U mobilized occurred during the first treatment.

The mobilization experiments indicated that soil peroxidation had the potential to release sorbed U from soils and soil matrix materials. The degree of U mobilization depended on several factors such as soil type, oxidant concentration, contact time and number of treatments. However, soil composition (organic content, amount of calcite) and treatment solution PH appeared to be critical factors controlling U mobilization.