**3. Effect of the parameters of water on heavy metal removal**

There are various water quality parameters that play an important role in the removal of heavy metals. The most important among them are; pH, temperature, natural organic matter (NOM), and ionic strength.

### **3.1 Effect of pH**

The pH of the water source significantly affects the presence of heavy metals and their properties. The formation of heavy metals in water sources is primarily affected by pH. Heavy metals are cationic in nature at neutral to low pH and possess more solubility and mobility in an aqueous solution. pH also affects the surface charge, ionization state, and the concentration of ions on the functional groups of the adsorbent [18]. Several studies have reported the effect of pH on the formation of heavy metals and their removal. The stability and mobility of copper in an aqueous solution have been reported to increase with a decrease in pH value [19].

With the increase in pH, heavy metals form complexes with hydroxyl ions which in turn affect the oxidation state of the heavy metals. Heavy metals tend to form solids when the pH of the aqueous solution exceeds its neutral value and gets precipitated from the solution. The oxidation state for chromium (Cr) in its stable form is Cr(III), and it changes to Cr(IV) with the increase in pH in this oxidation state, chromium has been found to be more toxic [20]. Lower pH (<4) increases the concentration level of H+ ions in the aqueous solution, which interferes with soluble metal ions and adsorbent surface interaction, thus reducing overall heavy metal removal [21, 22]. However, with the increasing pH (between 5 and 7), adsorption increases, and the adsorbent surface becomes more negatively charged and interacts readily with the positively-charged heavy metals [23].

The probability of removing maximum heavy metal ions by adsorption is minimal at lower pH values (<3) [24]. Furthermore, with the increase in pH, the concentration of H<sup>+</sup> ions is decreased, and a greater number of adsorption sites become available for heavy metal ions for adsorption, which thereby increases the heavy metal removal from water sources [18]. In the case of chromium removal, it becomes anionic in nature as the pH increases (e.g., HCrO4 − , CrO4 2−). Hence, the adsorption of chromium has been shown to decrease with the increasing pH of the solution. This is mainly due to the electrostatic repulsion resulting from negative surface charges on the adsorbent, inhibiting chromium adsorption [25, 26]. Overall, pH is a significant parameter that affects the behavior and removal of heavy metals from water sources.

### **3.2 Effect of temperature**

Temperature is another important parameter affecting heavy metals' behavior and subsequent removal from water sources. Mechanisms employed for the removal of heavy metals are enhanced at higher temperatures in which surface complexation reactions and various forms of ion exchange are also included [27]. It has been reported that an increase in the removal of Cr(VI) using hull wastes is achieved with an increase in the temperature from 5 to 40°C, which is attributed to the formation of additional adsorption sites on the surface of the adsorbent [28].

The adsorption process has been observed to increase with the increase in temperature due to the increased driving force of diffusion across the boundary layer and an increased rate of diffusion within the adsorbent [29]. However, in various cases, increased temperatures have also been observed to result in a decrease in the heavy metal removal from the water sources. For example, in one experimental study, the removal of total chromium by red algae has been reported to reduce from 90 to 78% with the increase in temperature, which has been possibly observed due to the tendency for ions to remain in the aqueous phase [30]. Furthermore, researchers have also reported a reduction in the heavy metal removal, such as Pb(II) and Ni(II) with increasing temperature, which was attributed to decreased surface activity [18, 31].

An experimental study had reported an increase in the adsorption of Cr(III) and Cu(II) onto peanut shells as the temperature increased to 50°C, and a decrease in

*Removal of Heavy Metals from Wastewater with Special Reference to Groundnut Shells… DOI: http://dx.doi.org/10.5772/intechopen.109904*

the adsorption when the temperature increased to 60°C, which has been observed due to potential damage to the adsorption sites on the peanut shells [32]. Another study evaluated a 32% decrease in the adsorption capacity of the olive cake with the increased temperature from 28 to 45°C in the removal of Cd(II) from an aqueous solution [33]. Therefore, it concluded with a fact that when estimating the effects of temperature on the removal of heavy metals from water sources, each adsorbent, and the corresponding metal ion must be explicitly evaluated in order to determine the overall impact of temperature changes on the adsorption process [13].

### **3.3 Effect of ionic strength**

The ionic strength of the water source has also been reported to significantly affect the removal of heavy metals. It has been observed that the presence of chloride in water sources can form neutral or negatively-charged chloride complexes that have a low affinity for adsorption. These complexes are soluble and difficult to remove from water. With the increase in the formation of chloride complexes, ionic strength tends to increase due to a decrease in the removal efficiency of Cu(II) and Ni(II) [34]. Researchers have also observed that increased salinity and increased concentration of dissolved metals, such as copper, cadmium, and zinc, possess a strong correlation [35].

Interactions between heavy metals and other surfaces are strongly affected by electrostatic forces, and increased ionic strength in a solution has a significant effect on the behavior and removal of heavy metals [13]. Zhang (2011) investigated the impact of ionic strength on heavy metal removal, including Cu(II), Pb(II), and Zn(II), by using dairy manure compost and reported a decrease in the removal of heavy metals with the increase in ionic strength [36]. However, various researchers have estimated that heavy metal removal increases with the increase in ionic strength. For example, Yang et al. (2016) reported a 25% increase in the removal of As(III) and Ni(II) with the increase in ionic strength from 0.01 to 1 M Cl<sup>−</sup> ions of the solution due to the formation of an inner-sphere surface complex [37].

### **3.4 Effect of natural organic matter**

Natural organic matter consists of humic and fulvic acids that are formed by the decomposition of plant and animal matter [38]. Natural organic matter is a complex mixture of organic compounds and is highly reactive with heavy metals, which alter the reactivity of the heavy metals and affect their mobility, bioavailability, and toxicity [38]. The particular impact of natural organic matter on heavy metals can be difficult to discover, primarily due to the wide mixture of additional factors, such as pH, humification of a particular natural organic matter, and oxidation state of heavy metals that contribute to the manner in which natural organic matter affects heavy metal removal [19]. Arsenic has been found to form complexes with both humic and fulvic acids, which leads to an increase in the immobilization of arsenic [39].

Metals, such as copper and zinc, have been observed to form complexes with natural organic matter [35]. An experimental study has reported that the removal of Cd(II), Pb(II), and Zn(II) by mollusk shells is increased by the presence of organic matter [40]. Moreover, the presence of natural organic matter can reduce the toxic form of chromium, Cr(VI), to its less harmful and more stable form, Cr(III) [19]. However, natural organic matter can reduce arsenic from its less toxic form, As(V), to its more toxic and mobile form, As(III) [19]. These research studies estimate that natural organic matter can reasonably affect heavy metal removal from water sources.
