Performance and kinetics of biochar for the treatment of landfill leachate containing COD and heavy metal pollutants

The study investigates the use of walnut shell-derived biochars, both pristine and engineered, for the treatment of landfill leachate containing chemical oxygen demand (COD) and heavy metals. While some countries, like Germany, have effectively reduced leachate pollution through strict waste pre-treatment and landfill engineering, persistent emissions of complex leachate remain a concern globally. To address the need for sustainable, cost-effective treatment options, this study evaluates biochar as an alternative to activated carbon in polishing applications. The research follows a multi-dimensional approach, starting with a systematic review of 207 peer-reviewed articles published between 2007 and 2023. The review provides a comprehensive meta-analysis of biochar application trends for the removal of heavy metals and organic compounds from aqueous media. Modified biochars accounted for 56% of the studies and consistently showed improved adsorption efficiency compared to unmodified types. The review also identified a research gap: most experiments were conducted on synthetic wastewater, with limited insight into biochar performance in real-world leachate systems. Furthermore, the interaction of co-contaminants such as metals and organics in landfill leachate has rarely been studied. To address this, walnut shells were selected primarily due to the availability of detailed characterization data, making them a promising candidate for further application-focused research; their lignocellulosic structure and prior use in industrial filtration also support their suitability as a biochar feedstock. Two additional feedstocks: pine cones and wood chips, were also examined for comparison. Biochars were produced using lab-scale thermogravimetric analysis (TGA/DSC 3+ Mettler Toledo GmbH, Gießen, Germany), a terrace oven Chantico (Sagawe & Sohn GbR, Hameln, Germany), and a full technical-scale reactor Biomacon Pyrolysis Boiler (Biomacon GmbH, Rehburg-Loccum, Germany). Physicochemical characterization revealed that walnut shell biochars exhibited low H/Corg (<0.2) and O/Corg (≤0.05) atomic ratios, indicating high carbon stability and aromaticity favorable for organic contaminant adsorption. Pine cone biochars showed the highest BET surface area (up to 598 m2 g-1), which correlated with greater capacity for heavy metal sorption. To enhance adsorption performance, walnut shell biochars were engineered through magnetization (Fe-WSBC-1) and humic acid impregnation (HA-WSBC-1), and evaluated under two separate experimental setups. In the first, WSBC-1 and Fe-WSBC-1 were tested for the removal of COD and five heavy metals (Cd, Cr, Ni, Pb, Zn) from biologically treated landfill leachate. Using Box-Behnken Design under response surface methodology (RSM), the effects of pH, contact time, and biochar dosage were optimized. Under optimal conditions, Fe-WSBC-1 achieved a maximum COD removal of approximately 35% (composite response), slightly higher than WSBC-1 at 30.6%. For heavy metals, Fe-WSBC-1 showed improved removal efficiencies across all target metals, with particularly notable results for Pb (93.8%) and Cd (89.1%). These findings predict the effectiveness of magnetization in achieving better sorption performance than its pristine biochar under complex wastewater conditions. To better understand the heavy metal adsorption behavior of walnut shell-derived biochars, competitive sorption experiments were conducted using WSBC-1, Fe-WSBC-1, and HA-WSBC-1 in both single- and multi-metal aqueous systems containing Cu, Ni, and Pb. Under single-metal conditions, all biochars show high removal efficiencies (RE) for Cu and Pb (RE > 97%), while Ni removal varies, with Fe-WSBC-1 achieving the highest capacity (4.45 mg g-1). In multi-metal systems, Pb maintains nearly complete removal (RE ≈ 100%) across all biochars. Cu removal declined under competition, with Fe-WSBC-1 showing 69.6% RE and 0.199 mg g-1 capacity. Ni showed the highest uptake in mass-specific terms, especially by HA-WSBC-1 (RE = 100%, qa = 1.998 mg g-1), followed by Fe-WSBC-1 (RE = 89.9%, qa = 1.795 mg g-1). These results highlight Fe-WSBC-1’s balanced performance and practical advantages, which were chosen for further adsorption model tests, while ATR-FTIR analysis confirmed mechanisms involving ion exchange, surface complexation, and redox interactions. Kinetic studies revealed that adsorption followed pseudo-second-order models for Ni and Pb, indicating chemisorption, while Cu showed poor model fit under competition. Isotherm analysis showed the Freundlich model consistently outperformed Langmuir, suggesting heterogeneous surface interactions; Pb maintained stable uptake, Ni showed strong multi-site binding, and Cu exhibited low affinity. pH tests confirmed increasing uptake of Cu and Ni with rising pH, reflecting electrostatic attraction above the pHPZC of 4.6 (Fe-WSBC-1), while Pb adsorption remained pH-independent, indicating specific complexation. ATR-FTIR analysis confirmed that –OH, –COOH, and Fe–O groups were responsible for metal binding, with Fe-WSBC-1 showing distinct peak reductions after Ni and Pb uptake, evidencing inner-sphere complexation. Together, the results confirm that Fe-WSBC-1 offers robust and balanced performance under complex multi-metal conditions, governed by both electrostatic and chemical interactions. The simultaneous removal of COD and heavy metals using WSBC-1 and Fe-WSBC-1 from real landfill leachate revealed important insights into potential adsorption interactions. For WSBC-1, the notably higher removal of certain metals compared to COD under identical conditions suggests competitive uptake, where active sites may be preferentially occupied by metal ions, limiting organic adsorption. In contrast, Fe-WSBC-1 demonstrated a more balanced performance, indicating that magnetization may mitigate such competition by providing additional or more versatile binding sites. These observations underscore the importance of considering co-contaminant interactions when evaluating adsorbent performance in complex wastewater matrices. A comparative evaluation of biochars from walnut shells, pine cones, and wood chips revealed that WCBC-2 (wood chip biochar, Biomacon) achieved the highest COD removal (80.63%) and adsorption capacity (8.57 mg g-1) at 200 g L-1, followed closely by PCBC-1 (giant sequoia cones) with 78.24% removal. Although WCBC-2 had a lower surface area (~247 m2 g-1) than PCBC-1, it still performed strongly, likely due to its moderate O/C and H/C atomic ratios (~0.09 - 0.12) and sufficient pore volume, enabling a balance between physical adsorption and interaction with polar organics. PCBC-1 benefited from high BET surface area (598 m2 g-1) and ash content (13.25%), enabling pore-filling and mineral-associated adsorption. In contrast, WSBC-2 (walnut shells) showed the lowest removal (52.35%), attributed to its low O/C ratio (0.03) and limited porosity, indicating that nonpolar interactions dominated but may be less effective for polar organic contaminants. The ranking of performance at 200 g L-1, where removal efficiencies were highest and most stable across all biochars, enabling a clear comparison of their maximum adsorption potential, was: WCBC-2 ≈ PCBC-1 > WCBC-1 > WSBC-1 > WSBC-2 > PCBC-2 (cones from pine trees). While wood chip biochars offered the most robust and balanced performance, cone biochars showed high adsorption via surface-driven mechanisms, and walnut shell biochars (despite their lower efficiency) remain promising for modification and upscaling. Beyond laboratory findings, the study proposes a closed-loop treatment concept that integrates biochar into landfill leachate management by utilizing the on-site compost sieve-overflow from biowaste and green waste as feedstock for biochar production. The resulting biochar can be applied (a) as an adsorbent for landfill leachate treatment targeting COD and heavy metals, (b) for soil improvement in the recultivation layer of landfill sealing by enhancing humus content and water retention under climate stress, and (c) as an additive in methane oxidation biofilters for treating landfill gas with methane concentrations below 12 Vol.-%. After use as an adsorbent, the biochar can be repurposed in landfill applications such as cover soil amendment or, if heavily contaminated, directed to thermal treatment as a final disposal step. In conclusion, this dissertation confirms that biochar, particularly in its engineered forms, offers a sustainable and versatile alternative to activated carbon for landfill leachate treatment. While activated carbon itself is a highly engineered form of biochar, engineered biochars, e.g., those magnetized or doped with functional groups, allow for greater tunability in targeting specific contaminants and integrating into circular treatment systems. Their dual functionality in adsorbing both organic and inorganic contaminants makes them well-suited for complex wastewater streams. In particular, biochar shows distinct advantages in targeting trace heavy metals due to its chemically active surface, an aspect less emphasized in conventional activated carbon applications. The study underscores the critical roles of feedstock selection, pyrolysis conditions, and surface modification in shaping biochar’s performance. Limitations of the current work include the lack of advanced characterization analysis by, e.g., following atomic-doped organic compounds and the absence of pilot-scale validation. Future research should focus on biochar regeneration, standardization of quality criteria, and its integration into broader waste valorization systems. Overall, the scientific results obtained provide a deeper understanding of the adsorption kinetics and the performance of biochar for the treatment of leachate containing COD and heavy metals, as well as for the practical implementation of biochar in the aftercare phase of a landfill and a sustainable circular economy.