- Alginate is an important part of the extracellular polymeric substances excreted by microorganisms giving rise to biofouling on reverse osmosis membranes in desalination plants for water treatment. We present a small-angle neutron scattering study on aggregation of alginate when exposed to an aqueous salt solution simulating the reverse osmosis concentrate of a wastewater reclamation plant. We explored the behavior of alginate as dissolved molecule and as grafted to the surface of gold nanoparticles thereby simulating the condition of alginate attached at the surface of a membrane. The behavior of dissolved and grafted alginate was different when exposed to the simulation solution. The dissolved alginate aggregates to a gel like structure whereas the grafted alginate induces formation of 0.2 μm large particles. These particles form stable composites within few seconds with volume fraction of about half mineral and half gold nano-particles.