# Adsorption of Hexavalent Chromium Ions Using Pine Sawdust Cellulose Fibres
## 1 Introduction  
Chromium (VI), a transition element, is naturally found in rocks and soil, existing in trivalent (Cr (III)) and hexavalent (Cr (VI)) oxidation states [1]. Its widespread use in various industries, including metallurgy, electroplating, and pigment production, resulted in its extensive presence in the ecosystem [2]. However, chromium (VI) is a highly toxic compound with potential health hazards, such as cancer, respiratory issues, skin irritations, and damage to vital organs [1]. Consequently, the removal of toxic chromium ions from water is imperative.  
Numerous techniques were explored for eliminating Cr (VI) from water, and, among them, adsorption stands out for its efficiency and cost-effectiveness [3; 4; 5; 6]. Agricultural-based materials garnered attention as alternative adsorbents due to their abundance, affordability, and eco-friendly nature [3]. Some such materials include ginger root, pineapple peel carbon, mucuna beans, maize cobs, rice husks, and tea leaves [7; 8; 9; 10; 11; 12; 13]. However, utilising untreated plant waste as adsorbents presents specific challenges, including the extraction of soluble organic compounds, the need for material swelling to increase surface area, and surface modification to enhance binding sites [14; 15].  
Various modifying agents, both mineral and organic, were used in pretreatment methods to enhance metal adsorption efficiency and eliminate soluble organic compounds and colouration from the solution [7; 15]. Past research explored diverse adsorbents forchromium (VI) removal, including activated carbon, chitosan, zeolites, and agricultural residues [16]. The current study focuses on a sustainable approach to chromium remediation from wastewater, utilising cellulose fibres (CF) as a promising candidate. CF exhibit unique properties, such as a large specific surface area, low density, reticular porosity, and hydrophilicity, making it readily modifiable for versatile functionality [17].  
Additionally, cellulose is widely acknowledged as an outstanding, economical, biodegradable, and recyclable resource in the environment. Numerous modifications were conducted on cellulose. For instance, [PERSON] et al., 2022 [18] developed a stable 3D network structure for cellulose fibres, employing various techniques and chemical modifications to enhance their ability to adsorb chromium (VI) from synthetic wastewater. [PERSON] et al., 2021 [19] conducted a study on the adsorption method that was deemed suitable for Cr (VI) removal, utilising an adsorbent synthesised from cellulose and polypyrrole. Additionally, [PERSON] et al., 2021 [20] synthesised Poly (m-aminobenze sulfonate) (PABS), a sulfonated polyaniline polymer known for its distinctive electroactive properties, thermal stability, and water solubility. Moreover, [PERSON] et al. conducted a study to extract cellulose from sisal fibre, employing chemical and mechanical treatments and grafting it with acrylic acid monomer, resulting in cellulose-g-acrylic acid as an adsorbent for Cr (VI) ion removal.  
The primary objective of this study is to explore cost-effective and efficient methods for chromium removal from wastewater, utilising environmentally friendly materials. Specifically, it aims to compare the removal capacities of untreated pine sawdust (UPSD) and chemically treated sawdust cellulose fibres (CF) using three two-parameter isotherm models. The investigation considers the effects of initial metal concentration, contact time, temperature, and pH.