A Method to Measure Arsenic Readily Released to Pore Waters from Uranium Mill Tailings John Mahoney Hydrologic Consultants, Inc., 143 Union Blvd., Suite 525, Lakewood, Colorado, USA Donald Langmuir Hydrochem Systems Corp., Denver, Colorado, USA John Rowson COGEMA Resources, Saskatoon, Saskatchewan, Canada

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Introduction Polymetallic U ores with As and Ni - in Raffinate Raffinate 300-700 mg/L As, 200-500 mg/L Ni, pH~1.0 Diffusion of As into Fox Lake required that As and Ni concentrations in tailings be around 1 mg/L Tailings neutralization circuit increases Fe:As >3 Neutralization with slaked lime to pH ~ 7

Tailings Neutralization Circuit to TMF 24 hour metallurgical composite sample Raffinate Tailings BaCl 2 Fe (SO 4 ) 3 CaO (pH 4) pH THICKENER NEUTRALIZATION TANKS Flocculant (pH 8) MIXING TANK Process Air

Mill and JEB TMF

Final Decommissioned TMF Glacial Till Waste Rock Tailings Mass Athabasca Sandstone

Tailings Optimization and Validation License Application - Initial model based on mineral precipitation (scorodite) and surface complexation of As on Hydrous Ferric Oxide (Langmuir et al., 1999) Demonstrated that As < 1 mg/L was possible Tailings Optimization and Validation Program (TOVP) ongoing studies to Verify and improve process, Monitor tailings management facility, and Validate model assumptions

Readily Released Arsenic Regulatory Agencies required - quantify amount of arsenic released from tailings via “desorption” process Surface Complexation Complicating factors Amorphous Arsenic bearing phases - Scorodite, Annabergite, Coprecipitates Need procedure that only targets “surface complexed” arsenic

Issues Discriminate As released through desorption while not releasing As in phases (Scorodite) Minimize time for dissolution Use a gentle extraction procedure Amorphous phases preclude extraction agents hydroxylamine hydrochloride, oxalic acid Spiking methods may change solution composition and require model calculations adding phosphate or molybdate

Methodology Developed to Measure Chlorinated Solvents in Soils - Portable Gas Chromatograph using Headspace Analysis Changing liquid/solid ratio changes concentration in solution Basically a Matrix Correction Serial Dilution Method Similar to Method of Standard Additions (MSA) Crawling Down the Isotherm

EPICS Procedure EPICS = Equilibrium Partitioning In Closed System Found Alternative Method in Gossett (1987) Calculate Henry’s Law constants

EPICS Method, No Sorption Es gibt nichts praktischeres als die Theorie - Boltzmann Pure water Pure water 3L 0.33 mg/L 2L 0.5 mg/L 1L 1 mg/L

EPICS Method, With Sorption Pure water Pure water 3L 0.45 mg/L 2L 0.655 mg/L 1L 1 mg/L As As

Example Calculation With Sorption

EPICS Plot Dilution Kd = 0/1 = 0

Features of Method Everything referenced to solution changes Requires solution concentrations only Matrix effect for soil water system Produces dimensionless Kd Suited for other hydrologic studies

Initial Modeling to Verify Procedure Simulated models using PHREEQC Defined Pore water and Surface - Diffuse Layer Model Defined Arsenic Free Solution Use MIX keyword to re-equilibrate with Surface Arsenic free leaching solution added in steps Changing proportions to mixtures Surface re-equilibrates with new solid/solution ratio Various simulation to cover range of conditions

PHREEQC Modeling

PHREEQC Based EPICS

Verification of the EPICS Procedure by Comparison with PHREEQC Modeling Results

Verification of Method PHREEQC models verified that overall approach and calculational method was correct Laboratory forms checked with PHREEQC results

Measurements on Tailings Models required constant major ion composition Surface charge effects may be important Maintain major ion concentration and pH Experimental Design 1Kg Slurry (tailings and pore water) Leach solution made from neutralized solution Very Small Sample Aliquots < 25 ml Dilutions ~ 1L arsenic free pore water/step 3 Steps - V4/V1 > 4 1 hour (12 hours) between extractions

Initial Experiments

Summary of Arsenic Desorption Measurements, One Hour Extractions V4/V1 is the overall dilution for the tests.

Long Duration Experiments

Summary of Arsenic Desorption Measurement Results - 12 Hours Extractions V4/V1 is the overall dilution for the tests.

Calculation of Readily Released Arsenic in JEB Mill Tailings using the EPICS Method

Percentage of Readily Released Arsenic in Mill Tailings

Conclusions - Modeling PHREEQC used to verify approach Simulation of experiments Assist in experimental design Check spreadsheets setup

Conclusions - Methodology Method is simple application of accepted technique Various approaches are essentially same method Straightforward method to measure distribution coefficient Should be applied to other systems

Conclusions – Arsenic in Tailings Amount of readily released arsenic in JEB TMF is very small ~ 0.2 percent of total Consistent with observation - adsorption of arsenic on ferrihydrite is not reversible Most Arsenic in Scorodite Suggests that adsorption of As is not occurring on other surfaces