Phosphorus Adsorption Capacity Evaluation for the Substrates Used in Constructed Wetland Systems: a Comparative Study

To provide a selection criteria to choose substrates according to the quality of the water requiring treatment in constructed wetlands (CWs), phosphorus (P) adsorption properties of four quartz sands with different physicochemical characteristics and four substrates commonly used in CWs were tested and evaluated via both the Langmuir and Freundlich equations. The effects of organic maters (OM) on P adsorption capacities of the substrates were also studied to evaluate the practical values of the substrates. The Langmuir sorption isotherm was used to estimate the P adsorption capacities of the tested substrates in CWs. Overall, the maximum P adsorption for all of the tested substrates decreased as follows: furnace slag > ceramic > zeolite > activated carbon > quartz quartz sands. The P adsorption capacities were significantly influenced by grain size; specific surface area; Fe, Al, Mg, and Ca contents in the substrates; and the amount of added OM. The chemical precipitation of P by the substrates containing Fe was more effective than those containing Ca at the same level. Added OM impacts P sorption by influencing pH, and substrates containing higher Fe and Al concentrations and lower Ca concentrations were more resistant to decreases in the P adsorption capacities due to OM accumulation. Finally, a principle for selecting substrates for the treatment of wastewater with different characteristics was determined.