To solve the water pollution problem caused by methylene blue (MB), areca residue biochar (ARB) was prepared by pyrolysis at 600 °C, and modified areca residue biochar (M-ARB) was obtained by modifying ARB with 1.5 mol/L NaOH, and they were utilized to adsorb and eliminate MB from water. The structural characteristics of ARB and M-ARB were examined, and the main influencing factors and adsorption mechanism of MB adsorption process were investigated. The outcomes demonstrated an increase in M-ARB’s specific surface area and total pore volume of 66.67% and 79.61%, respectively, compared with ARB, and the pore structure was more abundant, and the content of oxygen element was also significantly increased. When the reaction temperature was 25 °C, starting pH of the mixture was 10, the initial MB concentration was 50 mg/L, the ARB and M-ARB dosages were 0.07 g/L and 0.04 g/L, respectively, the adsorption equilibrium was achieved at about 210 min, and the elimination rate for MB exceeded 94%. The adsorption behaviors of ARB and M-ARB on MB were more in line with the Langmuir isotherm model (R² > 0.95) and the quasi-secondary kinetic model (R² > 0.97), which was characterized by single-molecule layer chemisorption. The highest amount of MB that may theoretically be absorbed by M-ARB in water ranging from 136.81 to 152.72 mg/g was 74.99–76.59% higher than that of ARB. The adsorption process was a spontaneous heat absorption reaction driven by entropy increase, and the adsorption mechanism mainly involved electrostatic gravitational force, pore filling, hydrogen bonding, and π–π bonding, which was a complex process containing multiple mechanisms of action. NaOH modification can make the ARB have more perfect surface properties and more functional group structures that can participate in the adsorption reaction, which can be used as an advantageous adsorption material for MB removal in water.

In line with the Sustainable Development Goals, a comprehensive understanding and precise quantification of the temporal and spatial characteristics of water quality is essential to identify sources of pollution in basins and to protect river headwaters to maintain water safety throughout basins. However, there is a lack of systematic quantitative tracking of seasonal river pollution sources in hilly areas in southwest China. In this study, the spatial and temporal characteristics of the water environmental factors in the Yaoshi River basin, the causes of pollution, and the main sources of nitrogen pollution were systematically investigated using geographic information systems and statistics. The results showed that the main pollutants in the river as a whole included five-day biological oxygen demand (BOD₅), oxygen demand (COD), ammonium (NH₄⁺), total phosphorus (TP), and total nitrogen (TN). Nitrogen pollution was the most serious, and various environmental factors caused seasonal variations and different spatial distributions in this pollutant. Temporally, the TP and COD concentrations were highest during the wet season, while the NH₄⁺, TN, and BOD₅ concentrations were highest during the dry season. Spatially, BOD₅, COD, and TP concentrations were ranked upstream < downstream < midstream, while TN and NH₄⁺ concentrations were ranked downstream < midstream < upstream. The main sources of pollution in the study area were domestic activities (34.94%), agricultural activities (40.12%), and mixed organic and natural sources (24.94%). Notably, manure effluent dominates nitrate nitrogen sources, while fertilizer inputs minimally contribute during mean-flow and dry seasons, and atmospheric deposition contributes during the wet season. These findings provide important data for improving water quality in the Yaosi River Basin, promoting sustainable agricultural practices, alleviating water scarcity, and advancing the region’s Sustainable Development Goals. In addition, this study can provide reference for the development of sustainable development management strategies for seasonal rivers in other hilly areas and promote broader environmental sustainability efforts.