###### Abstract  
Toxic heavy metals are ubiquitous in the aquatic environment and show a significant danger to human health. Carbon nanotubes have been extensively used in treating the contamination of groundwater due to their porous multi-layer nature. Batch tests revealed that oxidized multiwalled carbon nanotubes (O-MWCNT\({}_{5}\)) offer better removal of Pb(II). The removal rate of Pb(II) was 90.15% at pH 6 within 24 h, which was -58% more than that of Cd(II). The removal rate decreased to 55.59% for Pb(II) and to 16.68% for Cd(II) when the initial concentration of Pb(II)/Cd(II) ranged from 5 to 15 mg\(\cdot\)g\({}^{-1}\). The removal rate in the competitive tests was about 60.46% for Pb(II) and 9.70% for Cd(II). The Langmuir model offered better description of the adsorptive data for both ions. And the Q\({}_{m}\) of Pb(II) was 5.73 mg\(\cdot\)g\({}^{-1}\), which was 2.39 mg\(\cdot\)g\({}^{-1}\) more than that of Cd(II) in a single-icon system, while Q\({}_{m}\) was 7.11 mg\(\cdot\)g\({}^{-1}\) with Pb(II) and 0.78 mg\(\cdot\)g\({}^{-1}\) with Cd(II) in competitive water. And thermodynamic tests further indicated that the activating energy of Pb(II) and Cd(II) was 83.68 and 172.88 kJ\(\cdot\)mol\({}^{-1}\), respectively. Lead and cadmium adsorbed on the surface of O-MWCNT\({}_{5}\) are antagonistic in the competitive system. Based on XPS analyses, it was concluded that the absorbed lead/cadmium species on O-MWCNT\({}_{5}\) were (-COO)\({}_{2}\)Pb, (-COO)Pb(-O)/(-COO)\({}_{2}\)Cd, and (-COO)Cd(-O). Additionally, they offered theoretical evidence supporting the practicality of using nanocomposite membranes as a means to remove cadmium and lead. multi-walled carbon nanotubes; adsorption; oxidation; Cd(II); Pb(II) 2024 14 1745 122 economical and effective treatment methods to mitigate the severe heavy metal hazard to human health and safety [7]. In recent studies, scientists have investigated lots of methods for eliminating Pb(II) and Cd(II), for instance, ion exchanges, chemical precipitates, reversed seepage, membranes processes, microbiological biotechnology, coagulation, flocculation, and filtering and adsorption technologies [8; 9]. Nevertheless, the technologies have several disadvantages, namely, a low efficiency, a high cost, a hazardous co-product induction, operational delays, pollutant-specific inefficiencies, and a treatment process complexity [10; 11]. Among these techniques, adsorption is considered to be one of the most effectively and widely used techniques. This is primarily owing to its simplicity of operation management, potential for regeneration, cost efficiency, inertness to materials, absence of muck formation, and availability of a wide range of adsorbents [12]. In recent years, there has been a rising interest in the research on nanotechnology because of its unique physical properties, which may be attributed to their small dimensions, shapes, sizes, large surfaces areas, crystallization, and contents. In addition, nanocomposite membranes are considered to be the future of membranes and have demonstrated their remarkable capacity to adsorb heavy metal ions. This ability is greatly influenced by the careful selection of inorganic materials based on their physical properties [13; 14]. From a previous review of studies on the removal of toxic metals using carbon nanomaterials, the maximum adsorption capacity for the removal of toxic metals using carbon nanomaterials is in the following order: carbon nanotubes > graphene > activated carbon > carbon dots [15]. In nanomaterials, multi-walled carbon nanotubes (MWCNTs), innovative carbon nanomaterials, have been widely used in the treatment of water contaminated by heavy metals [16]. MWCNT\({}_{\text{S}}\) are known for their exceptional chemical stability, uniform surface distribution, and porous multilayer structure. However, MWCNT\({}_{\text{S}}\) tend to agglomerate rapidly, which leads to an obvious decrease in their adsorption efficacy.