## 1 Background
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Techniques** & **Materials used** & **Advantages** & **Disadvantages** & **Ref. ** \\ \hline \multirow{4}{*}{adsorption} & Natural adsorbents, Carbon & Simple, cost effective, efficient, & Expensive, low selectivity, & \\  & based hybrid adsorbents such as & reversible, environment & time consuming, difficult & [47; 7; 2; 73] \\  & carbon nanotubes, graphene, & & friendly. & sludge separation etc. & \\ \hline \multirow{4}{*}{Solvent Extraction} & Extractants, such as sodium & Efficient at higher & Complex, require continuous & \\  & succinate, laurly ammonium & concentration, better recovery & energy consumption, need & \\  & compounds etc. & of metals etc. & chemicals, inappropriate & \\ \hline \multirow{4}{*}{Ion Exchange} & Organic-inorganic or hybrid ion & Simple operation, high & Expensive, need diluted & \\  & exchanger as \(-\)SO\({}_{2}\)H and & \(-\)COOH based resin, & treatment capacity, high & medium, highly sensitive to \\  & salicylic-melamine-formaldehyde & efficiency, fast kinetics, able to & pH changes, difficult resin & [18; 76; 77; 78] \\  & resin etc. & clean up to ppb level etc. & regeneration etc. & \\ \hline \multirow{4}{*}{Membrane} & Polymer, carbon-based materials & No use of chemicals, ease of & Expensive, susceptible to & \\  & embedded into polymers/ & operation, hybrid membranes & fouling, continuous energy & \\ \cline{1-1}  & & comines separation and & consumption, high pressure, & \\ \cline{1-1}  & materials, etc. are used for & adsorption etc. & need diluted medium, Low & \\ \hline \multirow{4}{*}{Chemical Precipitation} & \multirow{4}{*}{Precipitating agents as lime, & Efficiently able to clean up to & Complex, require toxic and & \\ \cline{1-1}  & pyrite, sulfides, chelating agents. & ppm level, selective metal & & \\ \cline{1-1}  & & precipitation is possible. & explosive chemicals, cost & \\ \cline{1-1}  &  
Carbon (carbon nanotubes (CNTs), fullerenes, graphene, and graphene derivatives) based adsorbent materials are being used for the effective removal of the heavy metal ions because of their extraordinary high surface area to volume ratio and light weight as compared to other materials [66; 96; 97]. Currently, adsorbents based on the carbon nanotubes [98; 99], activated carbon [100; 101; 102; 103; 104; 105], graphene/ graphene oxide (GO) [60; 67; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117]; graphene magnetite's [118; 119; 120; 121; 122; 123] polymeric adsorbents [124; 125; 126; 127; 128; 129; 130; 131; 132] and other type of adsorbents [133; 134; 135; 136; 137] have attracted substantial attention for the removal of heavy metal ions from the wastewaters. Carbon nanotubes have significant adsorption capacities for the effective removal of heavy metal ions from aqueous solutions as compared to non-functionalized and activated carbon-based adsorbents [138; 139; 140; 141; 142]. Although, carbon nanotubes-based materials are the promising adsorbents but their high cost and lower availability, limit the large-scale applications as adsorbents. In addition to this, water permeation through CNTs membranes is a major concern which is still unclear even after both the experimentally and molecular dynamic studies. Therefore, economic, readily available, and effective adsorbents are the prime concern of the 21 st century for the effective removal of heavy metal ions from wastewaters. Although, the presence of humic substances has strong complexation binding ability with metal ions because of their abundant oxygen-containing functional groups, yet graphene-based adsorbents have been proven the best substituents for the removal of heavy metal ions from aqueous medium [66; 67; 143]. Therefore, the adsorption behavior of heavy metal ions onto the surface of graphene-based nanoadsorbents is the chief concern of this study. Graphene, a flat, single-atom thick sheet of sp\({}^{2}\)-hybridized carbon atoms with a two-dimensional honeycomb densely packed lattice arrangement, is an essential non-classical carbon adsorbent [144].