Water pollution by heavy metals, dyes, nutrients, pharmaceuticals and emerging contaminants is a serious global threat, and conventional treatment technologies are often energy-intensive, costly and unsuited to resource-limited regions. Cow dung (cattle manure) is an abundant, essentially cost-free feedstock for biochar, a carbon-rich porous sorbent. This critical review evaluates production routes (slow and fast pyrolysis, gasification, hydrothermal carbonization), activation and composite strategies, physicochemical properties, removal mechanisms, regeneration, cost and life cycle assessment (LCA) of cow dung biochars. Reported capacities, classified by contaminant and modification type and compared at the pH values given in Tables 1–4, span two orders of magnitude: ~6 mg/g for As(V) (pH 5–7) and ~12 mg/g for perfluorooctanoic acid (PFOA; pH 4–6); 113 mg/g Pb2 + for raw dairy-manure biochar (pH 5) rising to 386.5 mg/g after H3PO4 activation (pH 5); and 466.5, 339.5 and 264.2 mg/g for Pb2+, Cd2+ and Hg2+ on a phosphorus-rich hydrochar. For dyes, KOH-activated biochar reached 312.8 mg/g for malachite green (pH 7.5). Advantages include negligible feedstock cost, mineral-driven precipitation and ion exchange, and a favorable LCA; disadvantages include the low surface area and high ash of unmodified chars, feedstock variability, reagent and waste burdens of chemical activation, nutrient leaching, and weak affinity for anionic and fluorinated species. Cow dung biochar compares favorably with clays and zeolites for divalent metals and with activated carbon on cost, but continuous-flow and regeneration data are scarce and no pilot- or field-scale evaluations were identified. Priorities include standardized multi-contaminant, real-wastewater and fixed-bed testing, techno-economic analysis, and safe management of spent biochar.
This study assessed a Passiflora ligularis -assisted Fe 3 O 4 @eggshell nanocomposite as a waste-derived adsorbent for Cr(VI) removal in an upward-flow fixed-bed column. Synthetic-solution experiments at pH 4 evaluated influent Cr(VI) concentration (50–100 mg/L), active bed configuration (5.0–8.5 cm), and flow rate (3–5 mL/min). Within the investigated operating range, higher influent concentration produced earlier breakthrough and lower dynamic capacity, whereas deeper active-bed configurations increased service time and adsorbent utilization. The lower flow rate delayed breakthrough and improved pre-breakthrough performance but reduced cumulative uptake. The highest dynamic capacity (5.65 mg/g) was obtained at 50 mg/L, 8.5 cm, and 5 mL/min. Among the complete-profile models, Clark provided the best agreement (R 2 = 0.9376–0.9942; RMSE = 0.0294–0.0832), while BDST showed a preliminary linear depth–service time relationship (R 2 = 0.9577). Reuse tests showed progressive performance loss over three adsorption–regeneration cycles, with capacity retention decreasing to 53.03% after the second regeneration. Fe and Ca monitoring revealed changes in material release, while post-column characterization showed preservation of the calcite–magnetite structure with moderate restriction of accessible porosity. In fortified tannery wastewater, dynamic capacity decreased to 1.94 mg/g and breakthrough occurred earlier than under synthetic-water conditions. These findings support further assessment of Fe 3 O 4 @eggshell nanocomposite for continuous Cr(VI) treatment.
In this study, magnetic Co-doped γ- Fe 2 O 3 nanoparticles were synthesized through a fast, low-temperature method, and equipment involved adsorption, magnetic separation, and advanced oxidation process (AOP) was designed and used in the removal of tetracycline (TC) from polluted water. The synthesized Co doped γ- Fe 2 O 3 nanoparticles showed high adsorption capacity on TC and were characterized as a high-performance Fenton-like catalyst for the degradation of TC. The prepared nanoparticles were characterized by using X-ray diffraction (XRD), Fourier transform infrared (FTIR), vibrating sample magnetometer (VSM) for crystalline structure, various groups attached to the nanoparticles, and magnetic properties. The surface morphology and cluster size was confirmed by scanning electron microscopy (SEM), whereas the functional groups or free radicals produced during the degradation process were detected by EPR (Electron paramagnetic resonance). The adsorption of TC was tested for various concentrations, pH and time durations. Adsorption and AOP are two effective techniques for the removal and degradation of TC from water. The maximum removal efficiency was 98% for the initial adsorption at 5 mg/L and 10 mg/L was observed, whereas a peak efficiency of 99.3% was achieved in the device designed for this research. The excellent removal efficiency after five reuses of Co-doped γ- Fe 2 O 3 nanoparticles confirms the exceptional stability and reuse performance. The expected adsorption mechanism for the TC adsorption was presented, showing that the attachments of TC to the nanoparticles are only on the effective sites of the adsorbents [57–60].
