INNOVATION March-April 2014

Samples of Reactive Media Used in PRBs for Metal Remediation Reaction Type Reactive Media

Zero valent iron (ZVI), iron slag

Precipitation (Eh)

Ferrous hydroxide, Ferrous carbonate, Ferrous sulphide

Precipitation (pH)

Limestone

Precipitation (biochemical)

Organic matter (leaf detritus, compost, wood mulch, sawdust, manure, hay, sludge)

Peat, Lignite (Brown coal), Fly ash

Organic matter (leaf detritus, compost, wood mulch, sawdust, manure, hay, sludge)

Adsorption

Aluminosilicates (Zeolite), Activated Alumina

Phosphates (Apatite)

Activated Carbon, Exchange Resins

An iterative approach can be applied to balancing technical requirements and cost-effectiveness during PRB design. First, individual design and site factors must be assessed for technical feasibility and cost. Critical design factors (described below) are then ranked for further study and consideration. Reactivity Seasonal variations, maximum influent concentrations and potential future changes are considered when assessing mixture reactivity and longevity. To evaluate their suitability, different reactive media mixes are bench-tested using site groundwater. Understanding the influent water quality for PRBs is critical. As a PRB matures, both intended and unintended precipitates begin to accumulate on the front face of the wall (this is known as wall face fouling), which causes permeability to decline. To address this, site water quality is evaluated during preliminary PRB design to identify local media that may be reactive competi- tion or fouling by other contaminants, cations or anions. PRB design can address reactivity performance concerns by incorporating residence time safety factors (e.g. decreasing media density and/or increasing PRB wall thickness). Permeability safety factors and/or structural improvements (discussed below) can also be used to maximize PRB media reactivity and longevity. Permeability To avoid significant changes in groundwater flow patterns, the hydraulic conductivity of the PRB media is designed to be

The reactive media and treatment mechanisms are not mutually exclusive. For example, organic matter PRBs can also include limestone and zero valent iron (ZVI) to enhance biogeochemical precipitation of metal sulfides, while also promoting adsorption of metal contaminants onto both iron oxyhydroxides. Organic Matter, Limestone and ZVI PRBs Using sustainable, naturally occurring biogeochemical reac- tions, organic matter-based PRBs promote the growth of sul- phate‑reducing bacteria. The bacteria biologically reduces sulphate to sulfide, which then immediately reacts with metal contami- nants to form insoluble metal sulfides. Maintaining an anaero- bic, low Eh, pH-neutral environment is essential for promoting growth of sulphate-reducing bacteria and proper functioning organic matter/ZVI PRBs. Principles of PRB Design PRB design is a successful remediation method since con- taminated water must flow through and contact the reactive media. Successful PRB design depends mainly on the bal- ance of media reactivity, longevity, structure, and perme- ability. While longevity can be thought of as the factor that unites the three other design factors, all are interdependent and require careful planning and site-specific consideration in order to develop a successful PRB solution for effective ML/ARD treatment.

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