## 4 Conclusions  
The adsorption properties of four plant materials--_Melissa officinalis_ L. (MO); _Lavandula angustifolia_ L. (LA); and their respective waste materials (M and L)--towards Cu(II) ions were compared. The samples were characterized using SEM, XRD, BET, FTIR, and DTA/TGA. The optimal pH for adsorption was around four, with equilibrium reached within two minutes for LA, M, and L, and ten minutes for MO. Temperature and sample amount had negligible influence. Adsorption mechanisms were evaluated using Langmuir, Freundlich, and Dubinin-Radushkevich isotherm models, with non-linear chi-square (\(\chi^{2}\)) tests confirming Langmuir's suitability for MO, M, and L, while LA aligned better with the Dubinin-Radushkevich model. The maximum adsorption capacities were found to be 59.95 mg g\({}^{-1}\) (MO), 47.66 mg g\({}^{-1}\) (M), 40.52 mg g\({}^{-1}\) (LA), and 28.08 mg g\({}^{-1}\) (L), indicating that all materials have potential as effective sorbents for copper(II) ions. Functional groups such as carboxyl, phenol, alcohol, ether, and ester played a key role in the biosorption process. The highest adsorption capacity of MO correlated with more pronounced spectral variations. The waste materials retained sufficient active sites for effective sorption, achieving 80% for M and 70% for L as compared with the initial plant materials MO and LA, respectively. The thermodynamic analysis confirmed that Cu\({}^{2+}\) adsorption onto MO and M is an endothermic and spontaneous physisorption process, while for LA and L it is exothermic, with all biosorbents showing negative \(\Delta G^{0}\) values indicative of favorable and spontaneous adsorption.  
Desorption experiments with 1 M HNO\({}_{3}\) and 0.1 M EDTA demonstrated 100% recovery. MO was further tested for real wastewater purification, proving its reusability over four adsorption-desorption cycles with EDTA.  
Given adsorption's focus on innovative biosorption processes and sustainable applications, we believe our study will contribute to the future use of waste residues for environmental remediation.  
Figure 16: Results from four cycles of adsorption-desorption with eluent 0.1 M EDTA using biosorbent MO.  
**Author Contributions:** Conceptualization, L.I., P.V. and A.D.; methodology, L.I., P.V. and A.D.; software, L.I.; validation, P.V. and A.D.; formal analysis, L.I., P.V., A.D., V.K. and I.A.; investigation, L.I., P.V., A.D., V.K. and I.A.; resources, L.I.; data curation, L.I., V.K. and I.A.; writing--original draft preparation, L.I., P.V., A.D. and V.K.; writing--review and editing, P.V. and A.D.; visualization, L.I., V.K. and I.A.; supervision, P.V. and A.D. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
**Acknowledgments:** The authors express a special thanks to the \"Essential Oils and Herbs\" distillery in Zhelyo Voivoda village, Sliven district, for kindly providing us the plant samples after essential oil extraction. This work was supported by the European Regional Development Fund under the \"Research, Innovation and Digitization for Smart Transformation\" program 2021-2027 under the Project BG16 RFP002-1.014-0006 \"National Center of Excellence Mechatronics and Clean Technologies\"; National Scientific Infrastructure \"Energy Storage and Hydrogen Energy\" (ESHER), funded by the Ministry of Education and Science, contract No. DO1-349/13.12.2023. Research equipment of distributed research infrastructure INFRAMAT (part of the Bulgarian National Roadmap for Research Infrastructures) supported by the Bulgarian Ministry of Education and Science was used in this investigation.  
**Conflicts of Interest:** The authors declare no conflicts of interest.