Metal isotope complexation with environmentally relevant surfaces: Opening the isotope fractionation black box

Figures & data

Figure 1. Schematic representation of the isotope fractionation “black box”. Stable metal isotope fractionation occurs in the environment due to various biogeochemical processes taking place during the redistribution of metals within and between different environmental compartments. The resulting isotope signature can result from sorption/desorption, complexation, evaporation/condensation, redox changes, interactions with plants and microorganisms, etc. This review focuses on interactions of Cu, Cd, Hg, Ni, Zn isotopes with environmentally relevant mineral and organic surfaces. During these interactions between the aqueous and solid phases, the resulting isotopic fractionation is driven both by the aqueous speciation of targeted metals and surface properties of the corresponding bearing phases. The distribution of the metals among different aqueous species itself may lead to isotopic fractionation in the solution, which may subsequently affect the fractionation during surface complexation. At the same time, the type and density of active surface sites influence the nature of the complexes and subsequently the isotope fractionation factor.

Figure 2. Maximum values of Δ^114/110Cd after adsorption onto various surfaces (data from Wasylenki et al., 2014 were recalculated as Δ^114/110Cd = Δ^114/112Cd × 2).

Figure 3. Maximum values of Δ^65/63Cu after adsorption onto various surfaces.

Figure 4. Mass dependent Hg isotope fractionation during sorption on different solid phases from solutions of different Hg chemical forms. The ε notation is used specifically for MDF of Hg isotopes (ε^202/198Hg_solid-aq = δ^202/198Hg_sol – δ^202/198Hg_aq).

Figure 5. Maximum values of Δ^60/58Ni after adsorption onto various surfaces.

Figure 6. Maximum values of Δ^66/64Zn fractionation after adsorption onto various surfaces.

Table 1. Natural abundances (Rosman & Taylor, 1998) of selected metal isotopes and standard reference materials commonly used as isotopic reference points.

	Isotopes	Natural abundance (%)	Standard reference materials
			Name	δ value	Technique	Form	Reference
Cd				δ^114/110CdNIST SRM 3108
	106	1.25	NIST SRM 3108	0	MC ICP MS, TIMS	solution	Abouchami et al. (2013)
	108	0.89	JMC-Cd^o	−1.71‰	MC ICP MS	solution	Zhang et al. (2018)
	110	12.49	JMC-Cd Münster^o	−0.1‰	MC ICP MS	solution	Yang et al. (2018a)
	111	12.80	Spex-1 Cd^o	−0.1‰	MC ICP MS	solution	Zhang et al. (2018)
	112	24.13
	113*	12.22
	114	28.73
	116*	7.49
Cu				δ^65/63CuNIST SRM 976
	63	69.17	NIST SRM 976	0	MC ICP MS	solution	Maréchal et al. (1999)
	65	30.83	ERM®-AE633	0.01‰	MC ICP MS	metal bar	Moeller et al. (2012)
			ERM®-AE647	0.21‰	MC ICP MS	solution	Moeller et al. (2012)
			NIST SRM 3114	0.18‰	MC ICP MS	solution	Hou et al. (2016)
			JMC 310	0.95‰	MC ICP MS	solution	Petit et al. (2013)
Hg				δ^202/198HgNIST SRM 3133
	196	0.15	NIST SRM 3133	0	MC ICP MS	solution	Blum and Bergquist (2007)
	198	9.97	NIST SRM 8610	−0.58‰	MC ICP MS	solution	Donovan et al. (2013)
	199	16.87	ETH Fluka^o	−1.43‰	MC ICP MS	solution	Brocza et al. (2019)
	200	23.10
	201	13.18
	202	29.86
	204	6.87
Ni				δ^60/58NiNIST SRM 986
	58	68.08	NIST SRM 986	0	TIMS	metal bar	Gramlich et al. (1989)
	60	26.22
	61	1.14
	62	3.63
	64	0.93
Zn				δ^66/64ZnJMC-Zn-Lyon
	64	48.63	JMC-Zn-Lyon	0	MC ICP MS	Solution	Maréchal et al. (1999)
	66	27.90	NIST SRM 682	−2.45‰	MC ICP MS	metal bar	John et al. (2007)
	67	4.10	NIST SRM 683	0.12‰	MC ICP MS	metal bar	Yang et al. (2018b)
	68	18.75	IRMM 3702	0.27‰	MC ICP MS	solution	Doucet et al. (2016)
	70	0.62	AA-ETH Zn^o	0.28‰	MC ICP MS	solution	Archer et al. (2017)

*Natural radiogenic isotopes; ^osecondary standard materials.

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