Non-equilibrium zinc uptake onto compost particles from synthetic stormwater

► Zinc uptake from synthetic stormwater on various compost particle sizes was studied. ► Uptake on smaller particles conformed to equilibrium assumptions. ► Intra-particle diffusion caused rate limited metal uptake on larger particles. ► Non-equilibrium modelling adequately modelled larger particles. ► Design of bioretention systems should consider filtration media particle size. Zinc uptake onto different particle size compost was evaluated in batch and column studies using a synthetic stormwater to quantify sorption capacity and kinetics. The results showed that the pseudo equilibrium time for uptake increased from 2h to greater than 120h as the particle size of compost increased from 75μm to 6.75mm. This was due to intra-particle diffusion becoming a rate limiting process as the particle size increased. Column effluent data with 1.18mm particles could be fitted by Freundlich isotherm while that from the 4.75mm particles and a mixed particle size columns showed rate limited sorption with tailing and could not be adequately fitted using an equilibrium based isotherm. The results have established rate-limited sorption in amended filtration media due to larger particles under these flow conditions. This needs to be accounted for in the design of these filtration media and during performance modelling.