**2.1 Activated carbon adsorbents**

Activated carbon (AC) is one of the most widely used adsorbents due to its high efficiency, porosity and high surface area. It is commercially manufactured from the carbonization of like coal and wood, so it is expensive and its use is limited [24, 60–62]. They are mainly produced by pyrolysis of carbonaceous material at temperatures lower than 1000°C. The preparation of activated carbon involves two steps, one is the carbonization of raw material at temperature less than 800°C in inert atmosphere, second is activation of the produced product at temperature between 950°C and 1000°C [63]. Hence, most of the carbonaceous material can be used as raw material for activated carbon production, though the characteristics of the final product will rely on the raw material used and activated conditions [63]. Carbon is the main component of activated carbon adsorbent, other elements such as hydrogen, oxygen sulfur and nitrogen are also present. They are produced in both powdered and granular forms. The powdered one has large pores and smaller internal surface area; while the granular one has large internal area and small pores. The adsorptive capacity of an activated carbon is determined by its high porosity and surface are along with its chemical structure. Hence, other low cost raw materials

such as agricultural wastes are looked upon for increasing the cost effectiveness of activated carbon.

Kobya studied adsorptive removal of Cr4+ from aqueous solutions by AC prepared from hazelnut shell and reached a maximum removal of 170mg/g at pH 1.0 [64]. This was found to be higher than other adsorbents like coconut shell and wood AC [65] which had a removal of 58.5 and 87.6mg/g respectively. Karthikeyan et al. studied removal of Cr6+ from wastewater using activated carbon prepared from wood saw dust. The adsorption capacity of Cr6+ reached a maximum at 44mg/g at an optimum pH 2.0 [66]. This was significantly higher than other adsorbents for instance coconut shell carbon [67], treated saw dust derived from Indian rose wood [68], coconut tree saw dust [69] and sugarcane bagasse [70]. In these studies, the maximum adsorption was found to be 10.88, 10, 3.60 and 13.40 mg/g respectively. Kongsuwan et al. used eucalyptus bark for preparation of AC in the adsorption of Cu2+ and Pb2+ from low strength wastewater. The adsorption capacity for Cu2+ and Pb2+ was maximum at was 0.45 and 0.53 mmol/g, respectively [71]. El-Ashtoukhy et al. studied Cu2+ and Pb2+ removal from aqueous solutions by AC prepared from pomegranate peel. Batch adsorption experiments were conducted as a function of adsorbent dosage, contact time and pH. The removal of both the metals reached a saturation at 120 min with optimum pH 5.8, 5.6 for Cu2+ and Pb2+ [72]. Kavand et al. studied adsorptive removal of Pb2+, Cd2+ and Ni2+ from aqueous solution using granulated activated carbon. The removal was in the order Pb2+ > Cd2+ > Ni2+ at an optimum pH of 2, adsorbent dose of 2g/L and contact time of 80 minutes [73]. Kim et al. conducted a study on the removal of Zn2+, Ni2+ and Cr2+ from electroplating wastewater using powdered AC and modified powdered AC. A removal efficiency of around 90% was achieved for both the adsorbents at neutral pH [74].

### **2.2 Zeolites**

They are alumino silicates with a crystalline structure that occur naturally or are manufactured industrially. They are one of the best adsorbents for heavy metal removal as they consist of hydrated aluminosilicate minerals comprising of interlinked alumina and silica. Zeolites possess appreciable ion exchange capacities, hydrophilic properties and high specific surface area which makes then exceedingly good adsorbents for heavy metal remediation [75]. Zeolites can also be modified which attain a better adsorption capacity as compared to unmodified ones. NaX zeolite is one of the most widely used nanosized zeolite for removal of heavy metals from wastewater [76–79]. Rad et al. prepared NaX nanozeolite followed by polyvinylacetate polymer/NaX nanocomposite nanofibers to study removal of Cd2+. The maximum adsorption capacity was reported to 838.7mg/g at pH 5.0 [79]. Javadian et al. used fly ash for preparation of amorphous zeolite and obtained a maximum adsorption capacity of 26.246mg/g for Cd2+ at 5 optimum pH [80]. Similar studies were conducted by Visa who reported that zeolites have a high surface area and porosity which aid in adsorption of heavy metals [81]. Kobayashi et al. studied removal of Hg2+ and Pb2+ from aqueous solutions using zeolites prepared from fly ash. The maximum amount of Hg2+ and Pb2+ adsorbed were 22.4 mg/g and 30.7mg/g respectively at optimum pH of 5 [82].

#### **2.3 Clay minerals**

Bentonite, a clay mineral holds the highest cation exchange capacity, is regenerable and around 20 times cheaper than activated carbon [83, 84]. Clay minerals have less removal capacity of heavy metals when compared to zeolites. But they are still used owing to their advantages such as brilliant physical, chemical and

surface properties [84–87]. Jiang et al. studied removal of Ni2+, Pb2+, Cu2+ and Cd2+ from wastewater using kaolinite clay and it was found that concentration of Pb2+ decreased from 160.00 to 8.00 mg/L [88]. Bertagnolli et al. conducted a study on bentonite clay for removal of Cu2+ and achieved a maximum adsorption capacity of 11.89mg/g [89]. Chai et al. conducted a study using raw kaolinite and acid activated kaolinite for the removal of Ni2+ and Cu2+ from aqueous solutions and cemetery wastewater. The raw kaolinite adsorbed 69.23% Cu2+ and 63.37% Ni2+ whereas acid activated kaolinite adsorbed 77.47% and 68.32% at optimum pH of 7, contact time 60 min and temperature 25°C [90].
