Interim Report - Hanford Site

2.0 Laboratory Testing – Materials and Methods
2.1 Polyphosphate Hydrolysis Experiments
A long-chain polyphosphate molecule is required to forestall the hydrolysis reaction and release of
the orthophosphate molecule (PO 4 3- ). However, a balance between the rate of polyphosphate degradation,
groundwater flow rate, autunite/apatite precipitation, and injection rate must be met in order to optimize
the remediation strategy. Thus, a clear understanding of polyphosphate hydrolysis kinetics is necessary to
select the best chain or mix of polyphosphate chain lengths in order to directly precipitate autunite for
immediate mitigation of aqueous uranium concentrations, and further precipitate apatite to control the
long-term release of uranium from the sedimentary source.
In a homogeneous environment, the release of PO 4 3- is dependent upon both the chain length and the
pH of the solution; as the length of the phosphate chain increases, the hydrolysis rate decreases (Shen and
Morgan 1973). However, surface-mediated processes affect reaction rates in heterogeneous systems by
lowering the activation energy, E a , of the system, as expressed in the Arrhenius equation:
log k
+
=
⎛ − E
Aexp⎜
⎝ RT
a
⎞
⎟
⎠
(1)
where
k + = the rate constant
A = the frequency factor (also called the Arrhenius constant)
R = the gas constant (J mol -1 K -1 )
T = the temperature (K).
Therefore, it is essential to quantify the hydrolysis rates of long-chain phosphates in porous media before
a remediation strategy can be implemented effectively.
Aqueous cations are believed to accelerate the hydrolysis reaction for tripolyphosphate by
withdrawing the electron density from around the central phosphorus atom, thereby allowing nucleophilic
attack by water molecules more energetically favorable (Kura 1987; Kura and Tsukuda 1993; Shen and
Dyroff 1966; Watanabe et al. 1975; Wazer and Griffith 1955). Although much research has been
conducted on cation-accelerated processes, experimental conditions applicable to the Hanford saturated
zone have not been examined. The goal of this investigation was to elucidate the effect of aqueous
cations, pure minerals, and native Hanford Site sediment on the hydrolysis of tripolyphosphate using
phosphorus-31 nuclear magnetic resonance ( 31 P NMR).
The speciation of inorganic phosphate and its chemical affinity for other species in solution can be
readily assessed with 31 P NMR. Controlled 31 P NMR experiments were conducted to quantify the kinetic
degradation rate of the tripolyphosphate molecule under conditions present within the Hanford 300 Area
subsurface. The effect of aqueous cations (e.g., Al 3+ , Ca 2+ , Fe 3+ , Mg 2+ , and Na + ) and sedimentary
materials (e.g., FeOOH, and native Hanford sediment) on the hydrolysis of polyphosphates was evaluated
in potassium carbonate (K 2 CO 3 ) buffered solutions at 23°C. The K 2 CO 3 buffer was used 1) to maintain
the pH in a range near that of Hanford groundwater, pH = 7.5 to 8.5, 2) because carbonate is a major
component of Hanford subsurface and groundwater, and 3) because potassium has been shown to have a
low catalytic effect on phosphate hydrolysis (Wieker and Thilo 1960).
2.1