# Nickel (Ni\({}^{2+}\)) Removal from Water Using Gellan Gum-Sand Mixture as a Filter Material
## 4 Discussion  
This study shows an improvement in Ni\({}^{2+}\) adsorption using GG-treated sand. The difference in the amount of adsorbed Ni\({}^{2+}\) from the flows of 0.04 mL/min and 0.2 mL/min was determined by the GG content. To validate the practical applicability of GG-treated sand for heavy metal removal, the Ni\({}^{2+}\) adsorption capacity of 1 m\({}^{3}\) GG-treated sand with different GG contents at the flow of 0.04 mL/min has been evaluated as shown in Figure 10.  
The effect of hydraulic reduction with GG content decreases nonlinearly and levels off at a GG content of 1.0% [9]. However, if GG-treated sand is suggested for Ni\({}^{2+}\) treatment, 2% gellan gum can significantly increase Ni\({}^{2+}\) removal (Figure 9). To form 1 m\({}^{3}\) of the 2% GG-sand mixture, 26.5 kg GG should be dissolved in 463 L of heated water before mixing it with 1.3 tons of sand. If the GG-sand mixture is used to treat a 1 g/L Ni\({}^{2+}\) contaminated flow of 0.04 mL/min, 1906 kg of nickel can be trapped by GG. The experiment was conducted within a given time. Therefore, 2% GG could not reach equilibrium. The amount of adsorbed Ni\({}^{2+}\) by 1 m\({}^{3}\) GG-treated sand was calculated based on the experimental data. Therefore, the adsorbed Ni\({}^{2+}\) becomes higher in a field application.  
Figure 9: Potential maximal nickel adsorbability of specimens.  
Recently, bentonite has been widely used owing to its high number of specific surfaces for hydraulic barriers, effective restriction of the migration of solutes, and significantly high adsorption of heavy metals [30; 31]. However, bentonite causes side effects in humans (e.g., inhibiting hemolytic activity) and animals (e.g., damage to the lung epithelial cells of mice) [32; 33; 34]. A mixture including 10% bentonite and 90% sand is the most economical solution that satisfies the minimum requirement of hydraulic conductivity [35]. A mixture of bentonite-sand with a ratio of 1:9 (S9B1) was used as a 1 cm liner installed in the sand column. We observed that after 180 h subjected to the Ni\({}^{2+}\)-contaminated solution at a flow rate of 0.04 mL/min, the removal efficiencies of 10 BS, 2.0 HBL, 1.0 HBL, and 0.5 HBL were 60%, 53%, 36%, and 35%, respectively (Figure 11). The closest removal efficiency to that of 10 BS was 2% GG. Therefore, high GG can be used as an alternative material for bentonite.  
Figure 11: Comparison of the removal efficiency of pure sand, gellan gum-treated sand, and bentonite-treated sand.  
Figure 10: Potential nickel adsorption by 1 m\({}^{3}\) gellan gum-treated sand.  
The high cost of GG utilization for soil treatment compared with that of conventional hydraulic barrier materials such as bentonite is a severe concern for geotechnical engineers. For instance, 100 g of GG costs 217 USD [36], whereas 1 ton of bentonite costs approximately 75 USD [37]. However, owing to widespread usage in the food and cosmetic industries, the global GG market is expected to reach USD 62.96 million by 2023 from a market size of USD 50.257 million in 2017. Therefore, the use of GG for geotechnical application in general and for hydraulic barrier purposes is becoming increasingly feasible.  
This study aimed to remove a concerning heavy metal with two valence electrons, nickel (Ni\({}^{2+}\)), using GG-treated sand. One of the biggest advantages of the newly suggested GG-sand material is that it can be easily implemented in the field via direct mixing (e.g., GG-soil column mixing using an auger) or ground injection (e.g., GG solution grouting into porous ground). Implemented in situ, GG-treated sand layers (or barriers) are expected to enhance the ground strength in terms of shear strength and bearing capacity [14] in addition to reducing the ground water flow [9] and adsorbing hazardous Ni\({}^{2+}\) from contaminated ground water. However, in order to bring GG-treated sand into practice, further study needs to be considered for other types of heavy metals and actual wastewater that contains different kinds of chemical pollutants.