Table 1.

Overview of PSMs for metals

A - Overlying Water
Technique & metal	Description and operation	References (examples)
• Diffusion samplers (trace metals and major ions)	Simple diffusion cells are left in the water column to equilibrate with the ambient water. Equilibration time is dependent on the depth or design factor (F, cm) of the sampler device, with some samplers equilibrating in about 24 h in well-mixed waters (F = 1.0 cm) but others requiring 2 or more days.	(Benes and Steinnes 1974; Davison et al. 2000; Fortin et al. 2010)
• Diffusive gradients in a thin film (DGT) samplers (Al, As, Cd, Cu, Se, Hg, Ni, Pb, U, Zn; lanthanides)	Diffusion gradients in thin films give an estimate of “labile” metals, which are operationally defined by the nature and thickness of the gel through which the metals must diffuse, and by the strength of the binding phase. Kinetic regimen sampler, normally deployed from several hours to days. Use of a diffusion coefficient assumes no calibration is required.	(Davison and Zhang 2012; Dunn et al. 2003; Garmo et al. 2003; Zhang and Davison 1995)
• “Chemcatcher” samplers (Cd, Cu, Ni, Pb, Zn, Hg)	Teflon watertight body that retains a chelating disk; receiving phase is overlaid with a cellulose acetate diffusion-limiting membrane (0.45 µm pore size; 0.135 mm thick). Kinetic regimen sampler, deployed for periods of 1 to 4 weeks; requires laboratory calibration	(Aguilar-Martinez et al. 2011; Allan et al. 2008)
• “Gellyfish” samplers (Cu)	Polyacrylamide gel, embedded with iminodiacetate (Chelex) resin particles, is suspended in water until equilibrium reached; the amount of metal in the resin is proportional to the free metal ion concentration in the water. This technique has only been used in coastal marine systems. Deployment times of at least 8 d are needed to achieve equilibrium.	(Senn et al. 2004)
• Hollow fiber supported liquid membranes (Cd, Cu, Mn, Ni, Pb, Zn)	A hollow fiber is impregnated with a crown ether (ligand), dissolved in an organic solvent; the metal–ligand complex migrates across the fiber membrane and the metal is released to the aqueous acceptor phase in the lumen of the fiber. Can be used as either an equilibrium or kinetic passive sampler and provides the best estimation of free metal ions	(Bautista-Flores et al. 2010; Bayen et al. 2007; Parthasarathy et al. 1997; Sigg et al. 2006; Slaveykova et al. 2009)

B - Sediments
• Porewater peepers (Al, As, Cd, Co, Cu, Hg, Fe, Mn, Ni, Pb, Zn)	Multilayer diffusion cells inserted into sediments and allowed to equilibrate with the sediment pore water. Equilibration times range from about 1 day to well over 1 month (F = 1.0 cm), depending on whether the resupply of analytes from the sediment solid phase is able to sustain the pore water concentrations. The equilibration process has been modeled. Typical deployments of about 1–2 weeks are used.	(Brandl and Hanselmann 1991; Carignan et al. 1985; Hesslein 1976; Nipper et al. 2003; Teasdale et al. 2003)
• Teflon sheets (Fe, Mn + associated metals)	Teflon sheets act as a “substrate” for the deposition of iron and manganese oxyhydroxides and their associated metals; useful for identifying the boundary between the oxic and sub-oxic zones and adsorption of metals to new phases.	(Belzile et al. 1989; Feyte et al. 2010; Fortin et al. 1993)
• DGT samplers (As, Cd, Cu, Co, Hg, Ni, Pb, V, Zn)	DGT deployments within sediments are influenced by sediment response to depletion of analytes in the pore waters. The mobilization of metals from sediment solid phase can lead to pore water concentrations being fully or partially sustained. The response of sediment to DGT measurements has been characterized by several models, including interpretation of micro-niche responses. Metal pore water distributions can be determined at high resolution and in 2 dimensions.	(Davison et al. 2000; Davison et al. 1997; Stahl et al. 2012; Zhang et al. 1995)
• Diffusive equilibration in a thin film (DET) samplers (Cd, Cr, Cu, Fe, Mn, Mo, Pb, Re, U, Zn)	An equilibrium passive sampler that uses a thin layer of hydrogel as the sampling media. Deployed in unconstrained or constrained modes, with the former requiring fixing to minimize diffusional relaxation and the latter using compartments. Can be used to make measurements of 2-dimensional pore water distributions at high resolution.	(Davison and Zhang 1994; Fones et al. 2001; Krom et al. 1994; Yu et al. 2000)