This literature review investigated the efficacy of both unmodified and modified biochars as an adsorbent for phosphorus (P) capture from waste streams. The review focused on critical design aspects including the maximum phosphate removal adsorption capacities, influence of pH, coexisting ions, and pyrolytic conditions on P adsorption. The adsorption mechanisms, including electrostatic interactions, ion exchange, and complexation, were elucidated to provide insights into the underlying processes. The review revealed that the maximum phosphate removal adsorption capacity was influenced by factors such as the biochar type, surface properties, pyrolytic conditions, and presence of hydroxyl (–OH), carboxyl (–COOH) functional groups. The pH of the solution was identified as a crucial parameter affecting the efficiency of phosphate adsorption. Coexisting ions, notably carbonate anions, exhibited negative impacts on the phosphate removal process. Furthermore, this review discusses the characterization techniques used to assess biochar properties and adsorption mechanisms. Challenges related to biochar scalability, regeneration, stability, and metal leaching are addressed. Moreover, this review explored agricultural applications of P-laden biochar and found P-enriched biochar holds promise as a slow-release fertilizer for soil enrichment. This review highlights the role of biochar in promoting sustainable P management by integrating its application in wastewater treatment and agricultural reuse.

Soil moisture content is a critical parameter in understanding the water movement in soil. A soil moisture sensor is a tool that has been widely used for many years to measure soil moisture levels for their ability to provide nondestructive continuous data from multiple depths. The calibration of the sensor is important in the accuracy of the measurement. The factory-based calibration of the soil moisture sensors is generally developed under limited laboratory conditions, which are not always appropriate for field conditions. Thus, calibration and field validation of the soil moisture sensors for specific soils are needed. The laboratory experiment was conducted to evaluate the performance of factory-based calibrated soil moisture sensors. The performance of the soil moisture sensors was evaluated using Root Mean Squared Error (RMSE), Index of Agreement (IA), and Mean Bias Error (MBE). The result shows that the performance of the factory-based calibrated CS616 and EC5 did not meet all the statistical criteria except the CS616 sensor for sand. The correction equations are developed using the laboratory experiment. The validation of correction equations was evaluated in agricultural farmlands. Overall, the correction equations for CS616 and EC5 improved the accuracy in field conditions.

Soil monitoring plays an essential role in agricultural systems. Rather than deploying sensors' antennas above the ground, burying them in the soil is an attractive way to retain a non-intrusive aboveground space. Low Power Wide-Area Network (LPWAN) has shown its long-distance and low-power features for aboveground Internet-of-Things (IoT) communication, presenting a potential of extending to underground cross-soil communication over a wide area, which however has not been investigated before. The variation of soil conditions brings significant signal polarization misalignment, degrading communication reliability. In this paper, we propose Demeter, a low-cost low-power programmable antenna design to keep reliable cross-soil communication automatically. First, we propose a hardware architecture to enable polarization adjustment on commercial-off-the-shelf (COTS) single-RF-chain LoRa radio. Moreover, we develop a low-power programmable circuit to obtain polarization adjustment. We further design an energy-efficient heuristic calibration algorithm and an adaptive calibration scheduling method to keep signal polarization alignment automatically. We implement Demeter with a customized PCB circuit and COTS devices. Then, we evaluate its performance in various soil types and environmental conditions. The results show that Demeter can achieve up to 11.6 dB SNR gain indoors and 9.94 dB outdoors, 4× horizontal communication distance, at least 20 cm deeper underground deployment, and up to 82% energy consumption reduction per day compared with the standard LoRa.