Development of Efficient Novel Composites for Decontamination of Heavy Metals from Water and its Validation

In recent years, the existence of various harmful heavy metal ions in water bodies has become a major concern due to their persistent nature and detrimental impact on human well-being. The presence of arsenic, lead, and cadmium is a matter of concern as these contaminants are highly toxic and widely spread. The wastewater generated from different industries is the main reason for the contamination of different water systems by these toxic heavy metals. These metals do not degrade, and even at low concentrations, they can cause severe toxicity. These metal ions are notorious for their harmful impacts on the health of humans and other living organisms. The use of activated carbon to remove metal ions is widespread, but it has some shortcomings, like low adsorption rate, poor capacity, and limited selectivity in the presence of co-ions. Hence, there is a need for more efficient materials that can overcome these limitations. Despite the development of various composite materials in recent years, there are still some gaps in understanding their regeneration potential, real-world performance, and ability to remove both anionic and cationic metal ions. Most of the materials target the removal of individual cations or anions, there is a need for the development of materials that can target both cationic as well as anionic species. Moreover, most of the studies are concentrated on bench scale studies, as the real water contains other organic and inorganic ions along with these heavy metal ions studying the potential of the composite materials in natural/ real water systems will provide a better idea about the performance of composite materials. To fill these gaps, three materials were evaluated as potential solutions for heavy metal ion removal from water. The primary objective of the research was to create a composite material that could effectively eliminate lead, cadmium, and arsenic. Various metal-based composite materials were developed and tested for their potential use as adsorbents. These included MnO2-modified water caltrop peel activated biochar, MgO-modified humic acid-based magnetic composite, and reduced graphene oxide-Zn/Al LDH composite. The choice of biochar as the material for elimination was based on its porous structure and high specific surface area, making it an efficient solution for metal ion remediation. It can also be customized to meet specific application requirements. Water caltrop peel was chosen as the source material due to its availability and low cost, and its conversion into biochar provided a solution for disposing of waste biomass. However, the biochar material alone exhibited limited performance due to the absence of surface functional groups. Thus, it was further modified with MnO2 to enhance its removal efficiency. While it showed potential in removing lead and cadmium, it was inefficient in removing arsenic. This was attributed to the material's negative surface charge, which resulted in unfavourable electrostatic interactions with arsenate ions. However, the material effectively removed lead and cadmium, and further studies were conducted to determine its potential. The synthesized biochar composite exhibited a removal efficiency of 116.96 and 102.67 mg/g for lead and cadmium respectively at pH 6, contact time 30 min and adsorbent dosage 1g/L, which was higher than that reported in the literature for other biochar-based composites. The prominent mechanism for the removal of lead and cadmium by biochar-based material was electrostatic interactions and oxygen-complexation. One of the materials that was synthesized is the MgO-HA@Fe3O4 composite. Some research papers suggest that HA@Fe3O4 has the potential to remediate both cationic and anionic metal ions due to its abundant surface functional groups and electrostatic interactions. However, it has limited adsorption capacity. To enhance the material's adsorption capacity, magnesium oxide was utilized to improve its ion exchangeability. The synthesized composite material showed promise in removing lead and arsenic ions with an adsorption capacity of 248.76 and 104.17 mg/g at pH 6, contact time of 20 min and adsorbent dosage of 0.15 g/L, and it had a negative surface charge in the studied pH range, leading to the preferential adsorption of lead ions by electrostatic interactions. However, the material's ligand complexation interactions were crucial in adsorbing arsenic ions. The prominent mechanisms for the removal of lead and arsenic were ion –exchange, oxygen-complexation and precipitation. Despite having a negative surface charge, the material's removal efficiency towards cadmium ions was suboptimal. This could be due to the differences in metal ion characteristics and their affinity towards the binding site. Cadmium ions have a larger hydration radius than lead ions, and despite the material's negative surface charge, the large hydration size of cadmium can limit their accessibility to the adsorption sites, leading to lower efficiency for cadmium removal. Additionally, cadmium is a relatively soft Lewis acid with a lower surface charge density. Therefore, hard bases present on the material surface have lower efficiency for cadmium. The other material that was synthesized and utilized for metal ions removal was reduced graphene oxide based double layered hydroxide composite. rGO prevented the aggregation of LDH material and enhanced the surface area of the composite. The layered composite displayed high efficiency for the removal of lead, cadmium and arsenic with an adsorption capacity of 280.11, 227.27 and 178.25 mg/g for Pb(II), Cd(II) and As(V) respectively at pH 5.5, contact time of 45 min, and adsorbent dosage of 0.4 g/L. The layered structure of the material as well as its high ion exchange ability promoted the removal of both cationic (lead and cadmium) as well as the anionic (arsenate) species. The potential mechanism for the removal of metal ions involved was electrostatic interactions, ion –exchange, and oxygen-complexation. Water has a wide range of analytes and complex matrices. Thus, the concentration of elements varies from trace to percentage level in water starting from ground/surface water to tap water being used for household work. The present study is confined to real water samples from different sources like river/tank/tap water but not industrial wastewater. So, the validation study is carried out with the novel synthesized adsorbents to optimize their efficiency towards uptake of the contaminants up to a certain level. However, industrial waste is specific to the area with a very high concentration level of contaminants which will be a vast study. The materials displayed potentially good performance in real water samples displaying their potential for real world applications.The study found that rGO-Zn/Al LDH is the most effective material for removing lead, cadmium, and arsenic from water among the synthesized materials. However, further optimization of the synthesis conditions is still required to improve its performance. One possible approach for enhancing the material's removal capacity is through chemical modification, which can increase its surface area and functional groups. Further research is needed to optimize the synthesis conditions by varying factors such as temperature and time. This study provides insights into the mechanism behind the removal of lead, cadmium, and arsenic and suggests the potential of environmentally friendly adsorbents for metal ion remediation in water, which can support the development of more effective adsorbents in the future.