## 1 Introduction  
Dyes and pigments are found to be major water pollutants that contaminate surface and groundwater severely, and several of these synthetic dyes show mutagenic and toxic effects on human health. Dyes have a complex molecular structure that makes it difficult for them to degrade naturally; hence, they tend to remain in water bodies for long periods [1; 2; 3]. Many dyes are categorized as carcinogenic, which makes them harmful to the environment. Among the various classes of dyes, cationic dyes are a matter of concern, as most of them pose a severe threat to aquatic life, as well as to humans [4; 5]. Different types of artificial dyes are present in the effluent released from several process industries, particularly the textile, leather, and paper industries. These industries produce many kinds of dyes worldwide, with approximately 700,000 tons produced per annum [6; 7]. The discharge of colored wastewater into aquatic bodies not only degrades their aesthetic nature but it also hinders the transmission of light from the sun into water bodies [8]. Methylene blue (MB) is a well-known cationic dye that is widely used in the coloration of cotton, silk, and wool, and it may cause health problems such as nausea, vomiting, increased heart rate, tissue necrosis, eye burn, and difficulty in breathing [9; 10]. Therefore, it is essential to identify efficient methods to resolve dye pollution issues that are less expensive in terms of operation. To treat the wastewater before releasing it into water bodies, various treatment methods such as photodegradation techniques, activated sludge, and chemical coagulation are applied [11]. However, most dyes are non-oxidizable through traditional wastewater treatment. Nevertheless, dyes can adsorb onto adsorbents; therefore, adsorbents were found to be more efficient and, hence, attracted the attention of many researchers worldwide [12; 13].  
Recently, the application of nanotechnology flourished due to its immense benefits, attracting researchers to implement nanomaterials for water purification. This approach resulted in the high adsorption removal of pollutants in a cost-effective manner [14; 15]. Nanomaterials are appreciated for their high reactivity strength, low mass, and highly active surfaces [16]. Iron nanoparticles were successfully used for the adsorption of dyes such as methylene blue [17], crystal violet [18], phenol red [19], acid red 88 [20], and methyl orange [21]. There are various physicochemical methods used for the synthesis of zero-valent iron (ZVI); however, biogenic and green syntheses using plants attracted the attention of researchers due to their non-toxicity and reusability. Furthermore, plant extracts are able to stabilize ZVI and cap it. Several plant extracts were used for the green synthesis of ZVI and for the treatment of water, such as _Rosa damascene_ and _Thymus vulgaris_ extract applied for the removal of chromium(VI) [22], tea polyphenols applied for the removal of cationic dyes [23], and black tea extract for the adsorption of a second-generation herbicide anetryn [24].  
Hence, the aim of the present research study was to characterize and evaluate the surface adsorptive performance of zero-valent iron nanoparticles synthesized from sweet lime pulp waste (nZVISLP) to adsorb MB from wastewater. The process parameters affecting the process were studied scientifically. The optimal parameters of the isotherm/kinetic models that best represent the adsorption process were estimated using a hybrid evolutionary optimization technique. The novelty of this study is the application of nZVISLP for the removal of MB dye, as well as the application of a differential evolution optimization technique, which is a first in this field.