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Measurements of the stable isotope ratios 13C/12C and 15N/14N in cyanide in mine wastewaters can potentially be used to identify the mechanisms responsible for cyanide loss and to quantify the amount of cyanide that has been lost via a specific mechanism. Cyanide typically has large negative 5 13C values (VPDB scale) and near-zero 5 15N values (air scale) as it is initially applied to ores. Two cyanide loss mechanisms that occur naturally in mine wastewaters, evaporation as HCN gas and complexation with ferrous iron, have been found to fractionate the stable isotopes in laboratory experiments. Short-duration evaporation increased 5 13C and 5 15N values in the residual cyanide as would be predicted for a kinetically-controlled evaporation process. Longer-duration evaporation increased 5 15N but decreased 5 13C reflecting either liquid-vapor isotopic exchange or some degree of cyanide hydrolysis in the experiments. Complexation increased 5 15N slightly and decreased 5 13C slightly. The experimental results suggest that cyanide loss in industrial settings will also cause isotopic fractionation and thereby leave an isotopic signature in the residual dissolved cyanide. To fully evaluate the potential of isotopic tools, it will be important to carry out case studies at active and abandoned mines. INTRODUCTION Solutions of cyanide (CN~) are used at a large number of mines throughout the world to leach gold and silver from ores. The cyanide levels in these solutions decline during leaching and also during solution storage in surface ponds or tailings impoundments. A number of mechanisms are known to lead to cyanide loss, including evaporation, complexation with metals, oxidation, sulfidation, hydrolysis, precipitation as cyanometallic compounds, and bacterial processing (Smith and Mudder, 1991), but in case studies it can be difficult to identify the operative mechanisms or to determine their relative importance. The reason that the mechanisms are difficult to identify is that some of the important products are dispersed as gases or precipitated as solids. They are therefore difficult to detect and almost impossible to quantify. Cyanide loss from mine process waters and wastewaters is an important process to understand for two reasons. The first reason results from the fact that cyanide can be toxic (eg., Heming and Thurston, 1985). Natural degradation must be understood to realistically evaluate the environmental risks that would be associated with accidental release of process waters or with cyanide mobilization from tailings after mine abandonment. The second reason is that an understanding of cyanide loss can be used to minimize cyanide consumption thereby reducing mine operating costs (eg., Botz et al., 1995). A prerequisite for understanding cyanide loss is site-specific, quantitative information on the loss mechanisms. In this report we show how stable isotope tools may This report contains the text of a talk, slightly modified, that was delivered October 31, 1996 at the Geological Society of America meeting in Denver, CO. An abstract of the talk was published previously (Rye and Johnson, 1996). provide this information by (1) allowing the mechanisms to be identified, and (2) allowing the cyanide loss via a particular mechanism to be quantified. ISOTOPE THEORY Carbon and nitrogen, the constituent elements of cyanide, each has more than one naturally-occurring stable isotope. The behavior of the isotopes during cyanide loss can be illustrated by examining as an example one of the important mechanisms, evaporation as HCN gas. Figure 1 is a schematic diagram of the process. In aqueous solutions equilibrium is reached between the cyanide anion, CN~, and molecular HCN (figure 1). The position of the equilibrium is pH dependent. In active leach circuits where pH is about 10.5, CN~ is dominant, and in abandoned operations where pH drops over time, HCN becomes increasingly important. The sum of CN~ and is typically referred to as free cyanide. Approximately 98.5% of the free cyanide in solution will contain the common stable isotopes, carbon12 and nitrogen14. The remaining 1.5% will contain one or both of the minor isotopes, carbon13 and nitrogen15. Each of the different isotopic species shown in figure 1 has a slightly different equilibrium vapor pressure and evaporates at a slightly different rate. As a consequence, whether evaporation is a reversible equilibrium process or whether it is non-reversible and is governed by kinetics, the abundances of the isotopic species will differ in the gas and liquid phases (cf., Dansgaard, 1964). The differences will be on the order of parts per thousand or perhaps a few parts per hundred. The isotopic differences between the free cyanide and the evaporated gaseous cyanide can be expressed as fractionation factors (a) defined by the following equations for nitrogen and carbon, respectively, (15N/14N) in HCN gas aN (15N/14N) in free cyanide ( } (13C/12Q in HCN gas c ~ ( 13 C/ 12C) in free cyanide' ^ The effect of evaporation on the isotopic composition of the residual free cyanide can be predicted using the Rayleigh distillation model. The governing equation is Sfree cyanide = (Sfree cyanide-initial + 1000) F(«-D 1000 (3) where F is the fraction of cyanide remaining in the solution, 8 is the isotopic composition expressed as the deviation of 15N/14N or 13C/ 12C from the same ratio in a standard material in parts per thousand (%o), and a is defined as in equations (1) and (2) above. The Rayleigh model is shown graphically in figure 2 for a hypothetical case where cc=0.995 and 8free cyanide-initiai=0%<>. It is apparent from the figure that the 8-value for the residual free cyanide is driven upward with progressive evaporation. There is an equation parallel to equation (3) for the gaseous HCN that is produced. The result for gaseous HCN, which is also shown in figure 2, shows that the 8-value for the evaporation product is also driven upward. At any point in time (in other words, at any value of F), the gaseous HCN has a lower 8-value than the free cyanide by 5%o. The plot in figure 2 is the basis for quantifying cyanide loss. If the loss mechanism is known and if there is a fractionation factor characteristic of the mechanism, then the isotopic shift in either the reactant or the product can be used to determine the fraction of cyanide that has been processed. If the total quantity of cyanide in the system is known, then the quantity of cyanide lost and the quantity of products generated can also be calculated. Where more than one loss mechanism is operative, the overall effect on the isotopes will be a linear combination of the effects of the individual mechanisms. With respect to identifying loss mechanisms, our expectation is that the fractionation factors will differ from one mechanism to another. If this proves to be the case then one could, for example, analyze a solution repeatedly over a period of time and derive fractionation factors that would earmark the mechanism. There will be independent factors for nitrogen and for carbon, so the isotopic signature would not be a unique number but a unique combination of two numbers. EXPERIMENTAL RESULTS We have taken three steps in developing isotopic tools for use in cyanide studies. The first step was to work out analytical techniques because prior to our work the isotopic compositions of cyanides had never been measured. For solids we developed a combustion technique to form CC>2 gas and N2 gas for mass spectrometry (Johnson, 1996). For dissolved free cyanide we developed a method of precipitating the cyanide as a solid which can then be filtered, dried, and combusted. The second step was to analyze commercial compounds to determine the isotopic compositions of cyanide as it is initially applied to ores (Johnson, 1996). For six different compounds, 5 13C was found to range from -35 to -23%o on the Vienna Pee Dee Belemnite scale. The large negative values are in the range characteristic of sedimentary organic materials (figure 3). The cyanide isotopic compositions appear to reflect the carbon source that is used in the manufacturing process, normally natural gas or coal-derived coke (Kroschwitz, 1993). The 5 15N values are very close to zero on the air scale (figure 4). The nitrogen isotopic compositions appear to reflect the nitrogen source that is used in the manufacturing process, air (Kroschwitz, 1993). The third step was to carry out experiments to determine whether cyanide loss does in fact fractionate the isotopes. In one series of experiments HCN was allowed to evaporate into closed head spaces from solutions of sodium cyanide. The initial and final 5-values and the initial and final cyanide concentrations were measured, and the results were used to solve for the fractionation factors using equation (3). In a separate series of experiments we reproduced another important cyanide loss mechanism, complexation with ferrous iron. Solutions of sodium cyanide were prepared and different amounts of ferrous chloride were added to complex different proportions of the free cyanide as ferrocyanide. The initial and final free cyanide concentrations were measured as were the the isotopic compositions of the starting free cyanide and the complexed cyanide. Equation (3) was then used to solve for the fractionation factors. The results of the calculations are given in table 1. The fractionation factors for evaporation differed between the 7-day and 50-day experiments. The 7-day results indicate preferential loss of the light isotopes which is consistent with the kinetic control expected of an evaporation process. The 50-day results reflect preferential loss of light nitrogen and heavy carbon. The non-kinetic behavior of carbon may be explained by an approach to isotopic equilibrium between the dissolved and gaseous cyanide. The implication would be that carbon is more tightly bonded in the HCN gas molecule than in the CN~ anion, whereas ni

The use of stable carbon- and nitrogen-isotope data in quantifying cyanide loss from nine wastewaters