## 1 Introduction  
Due to its advantageous climate and natural landscape, Bulgaria is a suitable location for the growth of many medicinal plants, including _Melissa officinalis_ L. (lemon balm) and _Lavandula angustifolia_ L. (lavender). Both plants belong to the Lamiaceae family. Lemon balm is native to Central Asia and Europe [1], while lavender originates from the Mediterranean region, as well as southwestern and southern Europe, including Spain, France, Italy, Croatia, etc. [2]. In Bulgaria, wild leemon balm can be found throughout many parts of the country, although it has also become a popular cultivated crop more recently [3,4]. Our country is among the leading producers of lavender and its essential oils [2,5]. Lemon balm and Lavandula species serve a variety of purposes--e.g., in the food, pharmaceutical, perfumery, and cosmetics industries, as well as in aroma- and herbal therapy and even as natural insect repellents [2,4,5,6,7]. Since ancient times, essential oils from medicinal plants have been widely used in various sectors, including medicine, wellness, beauty, and food industries [5,8]. Lemon balm and lavender essential oils are used against numerous health problems and for boosting mood and immunity [4,5,7,9]. Both lemon balm and lavender essential oils, with yields ranging from 0.01-0.47% to 0.5-9.62%, respectively, have been widely studied for their chemical composition [3,4,8]. As demand for medicinal and aromatic plants rises, so do environmental challenges linked to improper waste management. Inadequate disposal practices increase public health risks, especially for communities near open dumpsites, due to issues like spontaneous fires, toxic emissions, and diseases [10]. Standard physicochemical methods for removing metal ions from aqueous solutions have a number of disadvantages, which necessitates the search for more effective alternative methods. One of these methods is biosorption, which uses biomaterials for purification and is considered an effective, innovative, reliable, economically advantageous, and environmentally friendly method for purifying aqueous solutions from toxic metals [11]. Recently, essential oil plants have gained interest as biosorbents due to their active phytocomponents, which can bind metals [12]. Using renewable or waste materials for biosorption can be more economical than conventional cleaning methods. Essential oil production generates substantial waste, prompting studies on the biosorption potential of these residual plant materials. However, no studies have yet explored the use of lemon balm distillation waste for toxic ion removal. Lavender waste has been used for dye removal [13], while immobilized distillation wastes from _Mentha arrensis_ L. and _Mentha spicata_ L. have proven effective in removing metals like Cu(II), Zn(II), and Pb(II) [14,15]. Rose petal distillation waste has also been used for Cu(II) and Cr(III) removal [16]. Additionally, [PERSON] et al. [17] examined chemically modified rose distillation sludge for adsorption of Pb(II) and Zn(II). Heavy metal contamination in the environment is a significant global issue due to the toxicity and long-lasting presence of these elements [12]. Copper is one of the most prevalent pollutants, often found in surface and groundwater, as well as in industrial wastewater. While copper is essential for human health, high levels can be extremely harmful, making it necessary to remove it from polluted water sources. Actually, copper is one of the most toxic metals for aquatic organisms, significantly more toxic than for mammals [18]. In the present study, two plant species, _Melissa officinalis_ L. and _Lavandula angustifolia_ L., denoted as MO and LA, and their waste materials remaining after essential oil extraction, denoted as M and L, respectively, were tested as biosorbents for copper(II) ions. Adsorption experiments were conducted to assess the effects of pH, contact time, initial ion concentration, sorbent amount, and temperature.