## 1 Introduction  
Nonylphenol (NP), a hydrophobic contaminant, has frequently been found in surface water, groundwater, sediment, and soil [1; 2]. It is a significant byproduct of a non-ionic surfactant, nonylphenol polyethoxylate, which is used as an ingredient of pesticides and personal care products, lubricating additive, the catalyst in curing agent of epoxy resin, defoamer in industrial laundries and dispersant in paper industries [2; 3; 4]. Nonyl phenol has a similar structure to natural estrogen, so it can mimic characteristic hormones by making a connection with estrogen receptors in the environment, which is highly toxic to fish, microorganisms, and aquatic plants. Additionally, the presence of NP in the human body will reduce sperm count and immunity and can cause breast and testicular cancers. Thus, NP has been listed as a priority substance in the EU Water-Framework Directive [5]. As a result, NP has been replaced by alcohol polyethoxylates in the most European, Canadian, and Japanese industries [1; 4]. However, alcohol polyethoxylates are expensive and less efficient in general, which boosts the illegal production and usage of NP. Moreover, inadequate NP removal by wastewater treatment plants (WWTPs) results in a high NP concentration in the aquatic environment [6].  
In general, the treated wastewater discharged from industrial parks in the Asian region including China, Indonesia, Korea, Laos, Malaysia, Taiwan, Thailand, and Vietnam contains a higher concentration of NP than that in the European Union, America, and Japan [6; 7; 8]. Table S1 summarizes the levels of NP detected in several major Asian rivers [6; 7; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28]. Generally, these rivers flow by industrial parks and WWTPs, which are the potential sources of NP. It is a great challenge to estimate the exact contribution by each industrial source to NP pollution of water environment, because of other significant sources such as municipal sewage, agriculture runoffs, and animal wastes [7; 29]. According to the Taiwan Construction and Planning Agency Ministry of the Interior (CPAMI), only about 50% of the municipal wastewater is treated in 2018 [30].  
Adsorption onto sludge particulates [5], biological treatment [31], chemical treatment [32], photocatalysis [33], electrochemical degradation [34], volatilization [5], and ozonation [35] have been applied for the removal of NP from wastewater. Among the technologies, adsorption has been considered as an important option because of the unique physicochemical properties of NP such as low water solubility and a high log K\({}_{\text{OC}}\) value [3; 36].  
Recently, carbon nanotubes (CNTs) have received much attention as an adsorbent because of its large surface area, highly porous and hollow structure, light mass density, and strong interaction with hydrophobic organic compounds such as NP [37; 38]. On the other hand, spent CNTs can be readily regenerated thermally, chemically, or biologically, so it can be readily reused.  
In the present investigation, NP in water samples, which were collected from source waters for 13 Taiwanese water treatment plants (WTPs), was adsorbed to modified multi-walled CNTs (MWCNTs); the kinetics and isotherm equilibrium of the adsorption process were studied. In particular, the influence of different experimental parameters, i.e., water pH, MWCNT dose, contact time, and temperature on the adsorption process.