# A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment
## 6 Factors Affecting the Adsorption Process
### Types of Adsorbent
#### 6.6.2 Lignin  
L Lignin is derived from black liquor and used as a devastating ingredient in paper processing units. [PERSON] et al. investigated the adsorption of zinc and lead on the surface of lignin [107; 121]. It is a very cheap adsorbent as compared to activated carbon for the elimination of toxic ions. The factor which was responsible for the sorption competence of lignin is polyhydric phenols and additional functionality present on the surface. The ion exchange method was used to remove heavy metal ions by the use of lignin. In a different experiment, [PERSON] and [PERSON] produced sulphuric acid-functionalised lignin that can adsorb mercury at concentrations of up to 150 mg/g [116]. [PERSON] and [PERSON] synthesised a natural sorbent from improved lignin material for the extraction of lead, cadmium, copper, and zinc metal solutions prepared in a lab with an initial concentration of 50 mg/L. To prepare this adsorbent, black liquor specimens were collected from the pulp industrial unit of the Mediterranean Foundation of SEKA in Turkey. The maximum adsorption value was found to be 11.3 mg/g for Zn (II), 17.5 mg/g for Pb (II), 7.7 mg/g for Cd (II) by the lignin. The time period to attain the highest adsorption was 4 h for lead ions with 96.7% adsorption capacity and for Zn (II) ions 10 h with 95.0% sorption efficiency. It was found that adsorption values improved with pH indicating that metal ions undergo ion exchange with lignin [122; 123]. Carboxymethyl lignin nanoparticles were prepared through a microwave heating method and antisolvent two-stage processes help to modify the extraction ability of carboxymethyl functionalised lignin and decrease the solubility of the resultant adsorbent. The adsorbent was used to extract lead ions and the maximum sorption value was found to be 333.26 mg/g compared to traditional adsorbents prepared from lignin. The effect of pH value was also studied in the range 2.04-7.06 for carboxymethyl lignin nanoparticles (CLNPs) as well as lignin nanoparticles (LNPs). It was found that with an increase in pH up to 6.03, CLNPs showed much higher adsorption compared to LNPs [124]. At lower pH values, the rate of sorption decrease more than LNPs due to the presence of a large number of H+ ions in the solution which combines with the -COO- group to form -COOH. Also, at pH value 6.03, carboxymethyl lignin nanoparticles showed better adsorption phenomena (217.21 mg/g) than lignin nanoparticles (51.61 mg/g). The regeneration ability of carboxymethyl lignin nanoparticles was better than lignin nanoparticles. Lignin/titanium oxide nanoparticles were synthesised for the extraction of metal ions as well as to study the photocatalytic features of the adsorbent [124]. In this paper, four types of lignin/nanomaterials such as corn stalks and cotton stalks, wood chips, and poplar wood chips nanotubes represented as L-TNTs CNL-TNTs, WL-TNTs, and PL-TNTs were used to extract lead, copper, and cadmium ions from the garbage. From the adsorption isotherm of materials, it was found that the extraction ability of WL-TNTs was more for lead, copper, cadmium compared to PL-TNTs. The highest sorption values were 677.6 mg/g 258.2 mg/g and 308.5 mg/g for Pb (II), Cu (II), and Cd (II), respectively by the WL-TNTs. PL-TNTs had shown 546.8 mg/g, 169.3 mg/g, and 245.4 mg/g for Pb (II), Cu (II), and Cd (II) extraction. An adsorption kinetic study showed that WL-TNTs rapidly removed metal ions Pb (II), Cu (II), and Cd (II) 87%, 71%, and 73%, respectively, within 5 min. After 120 min, there was no increase in metal ions adsorption. [PERSON] et al. prepared a novel environmentally friendly and cheaper adsorbent based on a magnetic core and mesoporous silica shell conjugated with the modified lignin (Figure 4) [125]. Lignin based-adsorbents were reviewed recently by [PERSON] and co-workers, with the focus on lignin, its modification, and carbon materials derived from this abundant feedstock [126].