Surface engineering of the lightweight aggregates derived from the dredged harbor sediment for green fertilizer utilization

Pursuing the circular economy driving scientists to explore additional environmental friendly application of lightweight aggregates (LWAs) derived from the dredged harbor sediment. We herein demonstrated the surface engineering of LWAs for the green fertilizer application by hydrothermally depositing iron species (Fe@LWAs) to increase the chemical reactivity. The obtained Fe@LWAs were first applied to adsorb nutrients (NH₄⁺ and PO₄³⁻). Desorption of the adsorbed nutrients was then utilized as the replacement of commercial chemical fertilizer to cultivate mung bean sprouts to further biofix environmental CO₂. Experimental results showed that the deposited iron species was the dominant component regulating the chemical behaviors of Fe@LWAs and adsorbed PO₄³⁻ played an important role for Fe@LWAs to adsorb NH₄⁺. With the coexistence of NH₄⁺ and PO₄³⁻, the adsorption capacities of Fe@LWAs were 0.220 mg-NH₄⁺/g and 0.486 mg-PO₄³⁻/g. The growth rate of mung bean sprouts using nutrient loaded Fe@LWAs was similar to those cultivated using commercial chemical fertilizer. Despite the nutrient recovery and mung bean cultivation decreasing the carbon footprint of the Fe@LWAs, the preparation of LWAs by high temperature sintering and the following hydrothermal surface modification consumed intensive energy. Consequently, the green fertilizer application of Fe@LWAs requires at least 594 cycles of nutrient recovery and mung bean cultivation to reach negative carbon emissions. Importantly, the cultivated mung bean sprouts could return to the soil to effectively secure the biofixed CO₂, which could contribute additional environmental implications by increasing the organic content in the soil at the same time.