Citrate is often used as a complexing agent to mobilize sorbed and precipitated uranium in both in situ and ex situ extraction of soils and nuclear reactor components. The biodegradability of U−citrate complexes is an important control over the potential migration of residual uranium after the extraction process is complete. In solutions buffered at pH 6−7, limited biodegradation of citrate is observed within 10 days with initial U:citrate molar ratios ranging from 1:2 to 1:8; however, over 99% of the citrate is biodegraded rapidly at pH 8−9. The increase of pH may have shifted the equilibrium speciation of uranium from (UO2−citrate)2 2- to (UO2)3(OH)7 1- and, consequently, raised the bioavailability of citrate. At pH 6−7, a significant amount of uranium is also observed to associate with biomass, whereas only a negligible amount is observed at pH 8−9. Our experimental results suggest that the residual concentration of uranium−citrate complexes left in the treated soils can be reduced rapidly if the soil water pH is held between 8 and 9 after the extraction processes.

Increasing water demand coupled with projected climate change puts the Southwestern United States at the highest risk of water sustainability by 2050. Membrane distillation offers a unique opportunity to utilize the substantial, but largely untapped geothermal brackish groundwater for desalination to lessen the stress. Two types of hydrophobic, microporous hollow fiber membranes (HFMs), including polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), were evaluated for their effectiveness in direct contact membrane distillation (DCMD). Water flux and salt rejection were measured as a function of module packing density and length in lab-scale systems. The PVDF HFMs generally exhibited higher water flux than the PTFE HFMs possibly due to thinner membrane wall and higher porosity. As the packing density or module length increased, water flux declined. The water production rate per module, however, increased due to the larger membrane surface area. A pilot-scale DCMD system was deployed to the 2nd largest geothermally-heated greenhouse in the United States for field testing over a duration of about 22 days. The results demonstrated the robustness of the DCMD system in the face of environmental fluctuation at the facility.

A vast quantity of produced water (3.8 million barrels in 2002) is generated during gas production in the San Juan Basin, New Mexico.Most of the saline produced water is re-injected into disposal wells at an average cost of $1-$4 per barrel.Due to the persistent drought condition in New Mexico, beneficial use of the produced water is receiving increasingly more attention in the gas production industry.Based upon the Resource Management Plan (2003) proposed by the Bureau of Land Management, about 10,000 new wells will be approved on federal land in the San Juan Basin for the next 20 years.Given the volume of fresh water required for hydraulic fracturing during well development, reclaiming produced water as the base fluid for fracturing not only may alleviate the impact of fresh water shortage, it also may lower the cost of well stimulation treatment. This project aims at 1) identifying the critical parameters and the corresponding optimal ranges in the laboratory, which will render produced water applicable as the base fluid for crosslinked gel-based hydraulic fracturing and 2) evaluating the validity of the laboratory findings under actual stimulation conditions in the field. Produced water from a gas-producing well near Aztec, New Mexico was used as the base fluid for the bench-scale experiments.Crosslinked gels were formulated with carboxymethyl hydroxypropyl guar gum (CMHPG), a zirconium-based crosslinker, persulfate breakers, and other ingredients.The apparent viscosity of the gels under different shear rates was measured using Fann Model 35A viscometers and the bottom-hole temperature simulated in the experiments was about 128 ºF.The rheological effects of several parameters, including pH, hydration time, ionic strength, and presence of sulfate, were evaluated.The experimental results demonstrated the feasibility of using untreated produced water from Aztec, NM as a base fluid for crosslinked gel-based hydraulic fracturing.Hydration time and pH were shown to influence the maximum apparent viscosity significantly in the absence of breakers.A positive correlation was observed between the maximum viscosity and the hydration time.For the solution pH, gellation at pH 5.5 exhibited the best rheological performance.By adjusting the hydration time, the gel loading, and the amount of breakers applied, it is conceivable that crosslinked gels with optimal rheological characteristics can be formulated with untreated produced water as the base fluid for well stimulations. A field test was performed following the bench-scale experiments to validate the laboratory findings under the actual stimulation conditions.A well located in San Juan County, New Mexico was restimulated using 70Q CO2 foamed 30-lb crosslinked gel.Untreated produced water was used as the base fluid for the crosslinked CMHPG gel.Natural gas production averaged 50 thousand standard cubic feet per day (MCFPD) prior to the stimulation and was increased to an average of 100 MCFPD after treatment indicating that the fracturing treatment using untreated produced water was successful in increasing the formation productivity.

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