Sludge-Based Superparamagnetic Nano-Sorbent Functionalized by Lanthanum Silicate Nanorods for Phosphorus Adsorption and Fertilization

Phosphorus (P) recovery from wastewater is considered to be a positive human intervention towards sustainable P use in the global P cycle. This study investigated the feasibility of synthesizing a superparamagnetic nano-sorbent that was functionalized by lanthanum silicate nanorods (NR_La-Si) using drinking water treatment sludge (DWTS), evaluating both its P adsorption capacity and fertilization effect. The DWTS-based La-modified P nano-sorbent (P-sorbent _D) exhibited complicated but single-layer-dominant adsorption for phosphate, with a maximum adsorption capacity up to 26.8 mg/g, which was superior to that of most of the similar sludge-based P-sorbent. The NR_La-Si-modified P-sorbent _D was identified with several characterization techniques and the leaching metal elements from the nano-sorbent were tested, which were below the limits proposed by the Food and Agriculture Organization of the United Nations. In addition, the growth and vigorousness of Arabidopsis thaliana indicated that the exhausted P-sorbent _D could be used as a potential water-soluble moderate-release P fertilizer, which was also confirmed by the well-fitted P uptake model and the P desorption pattern from the sorbent–fertilizer. The doped lanthanum silicate nanorods could play the dual role of P complexation enhancement and health/growth promotion. In light of this, this study proposed a new way of reclaiming DWTS as a P-sorbent for fertilization, offering new insights into the path toward “closing the P loop”.