Effect of Ionic Strength on the Solubility of Neptunium(V) Hydroxide

Abstract

The effect of ionic strength on the solubility of neptunium(V) hydroxide was studied under the oxidizing, C0 2 -free argon atmosphere in the pH range of 7 to 13.5 at room temperature (20-25° C). The solubility was mainly approached from oversaturation direction. The ionic strength was adjusted by NaC10 4 to 0.012, 0.05, 0.10, 0.40 or 0.80. Estimated values for the solubility product (K¡ρ) and the first and the second hydrolysis constants (ß u β 2 ) were:log K sp = -10.21 ± 0.02, log ßt = 2.91 ± 0.73, log ß 2 = 5.50 ± 0.06 at / = 0.80, log Κ sp = -9.61 ± 0.04, log jSj = 3.49 ± 0.25, log β 2 = 4.7 ±1.0 at 7=0.40, log = -8.94 ± 0.01, log β i = 2.67 ± 0.34, log β 2 = 5.74 ±0.07 at 7 = 0.10, log K sp = -8.91 ± 0.05, log ßi = 3.31 ± 0.30, log ß 2 = 5.74 ± 0.30 at 7 = 0.05 and log K sp = -8.04 ± 0.05, log ß 2 = 5.16 ± 0.94 at 7= 0.012, where 7 is the ionic strength in the molarity unit. By using these values, the specific-ion-interaction theory gives the constants at infinite dilution, 7= 0:log A? p = -8.68 ±0.26, log ßl = 3.30 ± 0.35 and log ß°2 = 5.58 ± 0.31.Neptunium hydroxide precipitates showed no X-ray diffraction peaks from 1 day to 3 months after preparation. This result indicates that the precipitates remained non-crystalline for up to 3 months.

Report on neptunium speciation by NMR and optical spectroscopies

“…The second interference, precipitation (equation 19), requires closer examination. ' Figure 14 shows solubility curves calculated from the thermodynamic hydrolysis constants of Itagaki et al (Itagaki et al 1992) and Neck et al (Neck et al 1992) These results are from studies reporting logpl values less than (Neck) or equal to (Itagaki) the value we have measured. Other reports with higher logpl values are discounted.…”

“…In fact, the ionic strength of 0.1 M was chosen because at low ionic strengths the solid formed in solubility experiments remains amorphous for several months (Itagaki et al 1992;Neck, Kim, et al 1992), and the solubility of neptunium is higher than at higher ionic stren,ghs. At room temperature we determined logPl= 3.40 k 0.22 for the equilibrium in equation 14 in 0.1M electrolyte.…”

“…A summary of the first and second hydrolysis constants taken from the literature for Np(V) is given in Table 1, and a graph of the first hydrolysis constant (logp1) as a function of ionic strength is shown in Figure 1 (Krause and Nelson 1948;Moskvin 1971;Sevost'yanova and Khalturh 1976;Schmidt, et al 1980;Maya 1983;Bidoglio et al 1985;Lierse et al 1985;Rosch et al 1987;Nagasaki et al 1988;Nakayama et al 1988;Itagaki et al 1992;Neck et al 1992). The first hydrolysis constant measured at 25OC is seen to vary by four orders of magnitude.…”

Near-field/altered-zone models report

“…The results suggested the product [NpO 2 + ][CO 3 2Ð ] decreases as the temperature is raised at a fixed Na concentration. Experimental and predicted Np(V) solubilities have been reported for concentrated NaCl solutions at 25°C in Runde et al (1996) and in salt solutions at ionic strength usually less than 1 (Neck et al, 1992(Neck et al, , 1994 (Itagaki et al, 1992;Robert et al, 1996). The results of the two studies are in agreement.…”

Fate of Contaminants in Contact with West Valley Grouts

“…Solubility of Np(V) hydroxide is sensitive to both pH and ionic strength (Itagaki et al, 1992). In the presence of Np(V) hydroxide, a minimum in Np solution concentration is found at pH 11 to 11.5.…”