Ananas comosus leaves were converted to a porous graphitized carbon (GPLC) material via a high-temperature pyrolysis method by employing iron salt as a catalyst. A cobalt molybdate (CoMoO₄)-and-GPLC composite (CoMoO₄/GPLC) was then prepared by engineering CoMoO₄ nanorods in situ, grown on GPLC. N₂ adsorption–desorption isothermal curves and a pore size distribution curve verify that the proposed composite possesses a porous structure and a large specific surface area, which are favorable for charge and reactant transport and the rapid escape of O₂ bubbles. Consequently, the as-synthesized CoMoO₄/GPLC shows low overpotentials of 289 mV and 399 mV to afford the current densities of 10 mA cm⁻² and 100 mA cm⁻² towards the oxygen evolution reaction (OER), which is superior to many CoMoO₄-based catalysts in previous studies. In addition, the decrease in current density is particularly small, with a reduction rate of 3.2% after a continuous OER procedure for 30 h, indicating its good stability. The excellent performance of the CoMoO₄/GPLC composite proves that the GPLC carrier can obviously impel the catalytic activity of CoMoO₄ by improving electrical conductivity, enhancing mass transport and exposing more active sites of the composite. This work provides an effective strategy for the efficient conversion of waste ananas comosus leaves to a biomass-derived-carbon-supported Co-Mo-based OER electrocatalyst with good performance, which may represent a potential approach to the development of new catalysts for OER, as well as the treatment of waste biomass.

Hainan, a well-known center of tropical agricultural production in south China, has received little attention regarding groundwater fluoride contamination. This study investigates the occurrence of fluoride in the western coastal area of Hainan Island and discusses factors affecting groundwater fluoride contamination in various aquifers and areas with different land-use types using hydrochemistry and multivariate statistical analysis. A total of 100 groundwater samples were collected from the western coastal area of Hainan Island. The results show that the groundwater fluoride concentration is as high as 4.18 mg/L and that F⁻-high (>1 mg/L) groundwater accounts for 9% of total groundwater. The proportion of F⁻-high fissure water is about two times that of F⁻-high pore water. Among the different land-use types, the proportion of F⁻-high groundwater from highest to lowest is as follows: bare land > cultivated land > woodland > construction land > grassland. The main factor affecting fluoride in pore water is the leaching of fluorine/aluminum-containing minerals such as phlogopite and calcite in the vadose zone, which is characterized by the co-enrichment of fluoride and aluminum in pore water. The leading cause of fluoride in fissure water is the leaching of fluorine-containing fertilizers, and continuous irrigation promotes the cation exchange of sodium, strontium, and calcium, which is characterized by the co-enrichment of fluoride with sodium and strontium in fissure water. Consequently, it is advised to minimize the excessive use of fluoride fertilizers and increase groundwater quality monitoring in order to decrease the emergence of F⁻-high groundwater in the western coastal area of Hainan Island.

Biochar materials exhibit promising prospects in adsorption–separation owing to their large specific surface area and well-developed pore structures. Herein, a series of manganese-oxide-modified porous carbon adsorbents (PEC-MnO_x) were fabricated via the pyrolysis method, using Ulva prolifera as the carbon source and KMnO₄ as the activator. XRD, SEM, TEM, N₂ adsorption–desorption, FT-IR and XPS were employed to characterize the morphology, crystal phase, pore structure and surface chemical properties of the as-prepared materials. The results demonstrated that KMnO₄ activation effectively optimized pore architecture, increased specific surface area, and generated abundant unsaturated –Mn–OH sites and oxygen vacancies. Batch adsorption experiments were carried out to evaluate Cd(II) removal performance. PEC-MnO_x-6 displayed the optimal adsorption capability with an equilibrium adsorption uptake of 152.4 mg/g at 298.15 K. Kinetic fitting suggested that the pseudo-first-order model better described the adsorption behavior. Intraparticle diffusion analysis confirmed the multi-step mass-transfer nature, and liquid-film diffusion acted as the primary rate-limiting step. Both Langmuir and Freundlich isotherm equations obtained high correlation coefficients, indicating the coexistence of homogeneous active sites and heterogeneous pores/defects on the carbon matrix. Combined with characterization results, Cd(II) sequestration was realized through synergistic effects mainly including surface complexation and ion exchange, with the assistance of electrostatic interaction. This work provides a reference for high-value utilization of Ulva prolifera biomass and remediation of Cd(II)-contaminated wastewater.