Hydrous oxide control model

Currently proposed licensing regulations for geologic nuclear waste repositories require a performance assessment involving long-term predictive capabilities. Previous work (J- 5) has shown the importance of solubility controls for modeling maximum actinide concentrations in repository groundwaters. However, until reliable data are available on the actinide solid phases that may be present or that may precipitate in the environment, the solubility of solid phases such as hydrous oxides that have fast precipitation kinetics can be used to initially set maximum solution concentration limits. 

Thus, the ability of the model to predict the chemistry of heavy metals in brine in a sense was used to test the validity of the carbonate subroutine. The general procedure was to assume that the trace metal solubility in brine was controlled by either the carbonate, basic carbonate or hydrous oxide form of the metal. The heavy metal and carbonate ion activities were determined by the model. The resultant calculated solubility of the heavy metals in brine was then compared with experimentally determined values. 

Many of the important chemical reactions controlling arsenic partitioning between solid and liquid phases in aquifers occur at particle-water interfaces. Several spectroscopic methods exist to monitor the electronic, vibrational, and other properties of atoms or molecules localized in the interfacial region. These methods provide information on valence, local coordination, protonation, and other properties that is difficult to obtain by other means. This chapter synthesizes recent infrared, x-ray photoelectron, and x-ray absorption spectroscopic studies of arsenic speciation in natural and synthetic solid phases. The local coordination of arsenic in sulfide minerals, in arsenate and arsenite precipitates, in secondary sulfates and carbonates, adsorbed on iron, manganese, and aluminium hydrous oxides, and adsorbed on aluminosilicate clay minerals is summarized. The chapter concludes with a discussion of the implications of these studies (conducted primarily in model systems) for arsenic speciation in aquifer sediments. 

Single Ion Activity Predictions. In lieu of an attempt to test the carbonate system predictive capability of the model, use was made of the fact that in most aerobic natural aquatic systems the solution concentration of heavy metal cations in the absence of interfacial phenomena is often controlled by the carbonate, basic carbonate or oxide (hydrous) solid phase form of the metal (, 33, 34). 

The authors found that the model-predicted As concentration is close to the leachate concentrations from the column packed with dust from the continuous reactor (1 120 H.gL-1 versus 1 330 xgL-1), when the solubility product of scorodite from Robins (1990) and the triple layer model is used. Only 11% As in the system was sorbed onto hydrous ferric oxide surfaces. Arsenic concentrations in the leachate are largely controlled by scorodite solubility. It should also be pointed out that simulations using solubility only and without including surface adsorption resulted in a closer match (1 270 n-gL-1 versus 1330 piglA1). For the simulation of the column experiments using wastes from the batch-reactor, the triple layer model predicted too low an As concentration (33 jigL-1 versus 120 pigL-1). 

Precipitation reactions cannot decrease dissolved arsenic concenPations below that in equilibrium with the solid. In contrast, the dissolved arsenic concentrations controlled by equilibrium sorption will decrease with increasing sorbent concenPation. This effect is illusPated in Figure 3, which shows the calculated disPibution of As(V) between sediment and porewater where this disPibution is controlled by As(V) adsorption onto hydrous ferric oxide (HFO). Increasing the concenPation of iron (present as HFO) in the sediment from 1 to 1.5 mg/g significantly decreases the predicted concentration of As(V) in the porewater. This modeling follows the approach used by Welch and Lico (33) except that we have used a published constant for the sorption of silica on ferrihydrite (114) rather than the estimated constants used by Welch and Lico (33). The choice of the... 

Figure 3 Dissolved As(V) concentrations in porewater ( X/L) as a function of the arsenic content of the sediment ( Xg/g) where partitioning between the solid and solution is controlled by sorption onto hydrous ferric oxide (HFO) with 1 or 1.5 mg/g Fe(III) (as HFO) present in the sediments. Modeling was performed using MINEQL (133) which incorporates constants for sorption onto HFO from Dzombak and Morel (134). Constants for sorption of silica from Hansen et al. (114) and of fluoride estimated by Dzombak and Morel (134) were included in the modeling. Calculations were performed at pH 8 with ionic strength fixed at 0.01. Dissolved concentrations of major ions were fixed at the values reported for Figure 2 (i.e., sorption of these ions onto HFO was not allowed to deplete their concentration in the porewater). Contents of As and Fe in the sediments were calculated based on Eq. (1) using D = 2.6 g Ian and ( ) = 0.3.

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