A Review of Adsorbents for Heavy Metal Decontamination: Growing Approach to Wastewater Treatment

Abstract

Heavy metal is released from many industries into water. Before the industrial wastewater is discharged, the contamination level should be reduced to meet the recommended level as prescribed by the local laws of a country. They may be poisonous or cancerous in origin. Their presence does not only damage people, but also animals and vegetation because of their mobility, toxicity, and non-biodegradability into aquatic ecosystems. The review comprehensively discusses the progress made by various adsorbents such as natural materials, synthetic, agricultural, biopolymers, and commercial for extraction of the metal ions such as Ni²⁺, Cu²⁺, Pb²⁺, Cd²⁺, As²⁺ and Zn²⁺ along with their adsorption mechanisms. The adsorption isotherm indicates the relation between the amount adsorbed by the adsorbent and the concentration. The Freundlich isotherm explains the effective physical adsorption of the solute particle from the solution on the adsorbent and Langmuir isotherm gives an idea about the effect of various factors on the adsorption process. The adsorption kinetics data provide valuable insights into the reaction pathways, the mechanism of the sorption reaction, and solute uptake. The pseudo-first-order and pseudo-second-order models were applied to describe the sorption kinetics. The presented information can be used for the development of bio-based water treatment strategies.

Keywords: adsorption; bioadsorbent; heavy metal; polymeric adsorbents; wastewater.

Figure 1

Number of research papers and patents available on the use of various materials for wastewater treatment per year from 2000 to 2020 from Scopus with the keywords “wastewater treatment”.

Figure 2

(a) General mechanism for the adsorption, (b) monolayer adsorption, and (c) multilayer adsorption.

Figure 3

Schematic representation of regeneration procedure of chitosan adsorbent. Reproduced with permission from [103].

Figure 4

A schematic process flow diagram illustrating the synthesis of the lignin-based hybrid magnetic nanoparticles. Adapted with permission from Reference [125].

Figure 5

Schematic representation of removal mechanism of chromium ions by chitosan-magnetite nanocomposite strip. Reproduced with permission from [137].

Figure 6

Adsorption mechanism diagram and contributions of adsorption Pb²⁺ and Cu²⁺ onto XCMCP [138].

Figure 7

The mechanism of Cd (II) and Pb (II) ions adsorption in montmorillonite and zeolites by ion exchange method. Adapted with permission from Reference [149].

Figure 8

Adsorption isotherm for metal ions adsorption by cyshtcc-Fe₃O₄, (a): Langmuir fit; (b): Freundlich fit; (c): Sips fit. Reused with permission from [155].

Figure 9

Desorption cycles of the poly (EGDMA-co-AAm) beads for Pb (II) metal ion. Reused with permission from [100].

Figure 10

Adsorption mechanism of PU. Reused with permission from [167].

Figure 11

Selective interactions among heavy metal ions and sulphur groups in covalent organic polymer suspended in water. Reused with permission from [228].

Figure 12

The structure of (a) MWCNT and (b) SWCNT [260].

Figure 13

Schematic representation of CNTs grafting and metals adsorption [268].

Figure 14

Synthesis routes of activated carbons from bio-waste.