A good method to remove heavy metal ions from the reaction mixture or product is a metal scavenging resin. The range of available heterogeneous scavengers is great and covers both neutral and ionic exchange resins, with either organic or inorganic backbones, and with general or specific uptake capabilities.

Perhaps less commonly used are scavengers for selected organic compounds based on complementary electrophilic-nucleophilic reactivity, e.g., aldehyde-amine and Michael acceptor-thiol interactions.

Adjusting pH during workup is a very common procedure. Ion exchange beads are an option for doing so and offer a strategy to produce less salt in the vessels during workup.

We support the notion to include the treatment of waste streams into the downstream processing of reactions. It may make environmental and/or economical sense to treat own waste streams locally before discharge or destruction. At the production site, the waste streams are contained, concentrated, and it is likely the place where the most knowledge about the materials involved is available. On the other hand, whenever large, costly, or specialised treatment equipment is needed, it may make more sense to transport wastes to a common waste treatment site.

Whether the waste stream contains excess dye from clothing/textile manufacture or active pharmaceutical ingredient (API) precursors from pharmaceuticals production, a simple activated carbon treatment may suffice to make the waste stream acceptable for discharge.

In the best case scenario, successful waste treatment can be conducted according to environmental and financial targets. It might even allow to re-use certain mass streams such as aqueous media or organic solvents. Ideally, waste streams can be turned into a resource by extracting residual amounts of valuable components. Two examples from the pharmaceutical industry:

1. Scavenging and re-use of transition metals (e.g., Pd, Ru or Ir) from reaction waste after transition metal-catalysis
2. Recovery of API:s and precursors to API:s in small, residual concentrations from reaction or purification step waste.

In both cases, the effluent would likely be prove harmful to, e.g., aquatic life, but both the transition metal and API precursor are valuable when effectively recovered. For this, metal scavengers can be added to the bulk liquid and filtered off in the next step. Likewise, it is possible to utilize columns packed with the scavenger or adsorbent/absorbent of choice. Both these operations might be tedious and associated with work health issues when handling the chemicals.

Another alternative is the rotating bed reactor (RBR). Filled with the solid phase of choice and operated in existing vessels or in external waste storage vessels, it can be deployed in a versatile and flexible manner.

Figure. Before and after the downstream processing of a palladium containing solution using metal scavenger in a RBR. Find the application details here.