## 4 L-MMCF Manufacturing Process
### Chelation and Bleaching
Aminopolycarboxylates such as ethylenediaminetetraacetic acid (EDTA) or diethylenetrinamepentaacetic acid (DTPA) are commonly used for this purpose. Both chemicals are optimal for metal ion removal without significantly reducing cellulose DP. Chelating agents bind to metal ions in a solution, allowing the metal ions to become water soluble and easily removed. There are multiple interaction sites in chelating agents where lone electron pairs donate an electron to the metal ions. In this interaction, the chelating agent binds with the metal ion, forming a water soluble chelating ring [109]. After chelation, the pulp is rinsed with distilled water to remove chelating agents and inorganic materials (i.e., metal ions). EDTA and DTPA are found in everyday household items, such as detergents, cosmetics, and paints [107]. Unfortunately, these chelating agents pose increased environmental risks due to solubilisation and leaching of heavy metals into water or soil [79]. As such, alternative biodegradable chelating agents are currently being evaluated, including pyridine-2,6-dicarboxylic acid (PDA), methylglycinediacetic acid (MGDA), nitrilotriacetic acid (NTA), ethylenediaminedisuccin acid (EDDS), and iminodisuccinic acid (IDS) [106; 107]. Due to increased awareness and understanding of manufacturing effects on environmental systems, adoption of more environmentally sustainable chelating agents must be addressed by the pulping industry. Bleaching is necessary to remove residual lignin and reach the desired fibre brightness. The bleaching process uses an elemental chlorine free (ECF) chemical in the form of chlorine dioxide (ClO\({}_{2}\)), denoted as \"D\" in bleaching sequences, or total chlorine free (TCF) chemicals in the form of oxygen (O\({}_{2}\)), ozone (O\({}_{3}\)), or hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) [10; 79; 110]. TCF bleaching, while effective, can react with very small concentrations of metal ions, specifically Fe, Cu, and Mn, resulting in chain cleavage and reduced DP [79; 111]. Effective bleaching of hemp best fibre can be achieved with a 5% H\({}_{2}\)O\({}_{2}\) solution at 85 \({}^{\circ}\)C with a 20% acetic acid washing step prior to and post bleaching, following by rinsing with deionised water [26]. Increased environmental regulations to reduce toxic chlorinated organic compounds in pulp effluents has led to improvements in both ECF and TCF bleaching technologies, with ECF providing a more economical approach [112]. Multiple steps of chelation and bleaching may be required. A common sequence is O-D1-Ep-D2, where O stands for oxygen delignification (immediately after cooking), D1 is the first ClO\({}_{2}\) stage (pH = 2-2.5), Ep stands for peroxide-reinforced alkaline extraction, and D2 is the second ClO\({}_{2}\) stage (pH = 4-5). Chelants are added at the Ep stage to bind any residual transition metals and keep them from catalytically decomposing peroxides. There is a displacement washing step between each stage, where clean water is added to displace the filtrate [113]. This filtrate contains all the compounds that have been extracted from the pulp (lignin, metals, etc.). With the acidic conditions of the first D stage, most metals become soluble and end up in the D1 filtrate. After bleaching, the pulp is washed and drained to remove all contaminants and reduce pH to neutral (note: a small amount of strong acid, such as sulphuric acid, is generally required for pH reduction). The pulp is then dried on screens to form a continuous sheet of dissolved pulp. Table 4 shows the metal scan of as-received hemp hurd and hemp best, as-prepared PHK pulp, and post chelating/bleaching. The metal scan was performed by the authors using ICP-OES. It illustrates the efficacy of chelating in the removal of metal ions. The reduction of Fe, Ca, and Mg content may also be possible by optimising the prehydrolysis process conditions or introducing additional bleaching sequences (e.g., acidic pre-treatment) [102].