Application of carbon-loaded solid waste as substrate in constructed wetlands: A comparative study of two carbon loading methods and their water treatment performance

• Carbon loading increased surface area, aiding microbial attachment and growth. • Both carbonation and HTC significantly enhanced water treatment performance. • Hydrothermal carbonization outperformed carbonation in N&P removal efficiency. • Carbon-loaded substrates promoted plant growth and microbial community proliferation. • Foamed concrete with hydrothermal carbonization showed superior pollutant removal. Steel slag and foamed concrete are generated in large volumes as industrial and construction wastes, respectively. Their high porosity and specific surface area contribute to favorable adsorption performance. However, their limited capacity for microbial attachment restricts their application in constructed wetlands and other water treatment processes. This study investigates the application of carbonation and hydrothermal carbonization (HTC) as modification methods to enhance the water treatment efficiency of steel slag and foamed concrete, two porous solid wastes with limited microbial attachment capabilities. Carbonation increased the BET surface area of foamed concrete from 9.02 m 2 g −1 to 21.53 m 2 g −1 and that of steel slag from 6.75 m 2 g −1 to 17.85 m 2 g −1 , whereas HTC further enhanced these values to 24.70 m 2 g −1 for foamed concrete and 19.13 m 2 g −1 for steel slag. Both treatments significantly improved phosphorus and nitrogen removal capacities, with HTC being more effective, achieving phosphorus adsorption up to 2.20 mg g −1 and nitrogen adsorption up to 1.82 mg g −1 . In simulated constructed wetland systems, both carbon-loaded pretreatment methods notably promoted plant growth and enriched microbial communities involved in nutrient cycling, particularly Alphaproteobacteria and Gammaproteobacteria, alongside Bacteroidetes. Pseudomonas species were markedly enriched in phosphorus removal processes, especially within MT1 and MT2 systems, showcasing superior phosphorus removal efficiencies. The findings indicate that carbon-loaded substrates, especially HTC-modified foamed concrete, create favorable microenvironments for denitrifying and phosphorus-removing microbiota. This work links wastewater purification, solid waste valorization, and carbon mitigation, supporting environmental protection and sustainable development goals.