Removal of toxic heavy metals from wastewater: recent updates and future outlooks on eggshell as a promising low-cost adsorbent

Heavy metal ions in wastewater pose a significant global challenge to environmental pollution, despite technological advancements. Proper treating of these heavy metals is crucial to prevent human health risks, making wastewater treatment a critical global issue. Since the toxic metals in the water could not be entirely removed by a conventional treatment system, new purification techniques employing low-cost materials were investigated to enhance the present treatment procedure. Various inexpensive adsorbents were employed to extract heavy metals from wastewater and aqueous solutions, such as agricultural, seafood, and food waste. This investigation aims to assess the techniques, benefits, regeneration, and future prospects of eggshells for toxic heavy metal adsorption in wastewater treatment, particularly chromium, cadmium, lead, arsenic, nickel, and mercury. This assessment summarises the characteristics that affect maximum adsorption capacity, such as pH, contact duration, temperature, initiating concentration, and adsorbent quantity. A thorough grasp of the kinetic uptake of the chosen adsorbents was also provided by comparisons between the modified and natural eggshell adsorbents. The major takeaways from this review are as follows: chemically or physically modified eggshells exhibit higher adsorption capacity, with calcined eggshell, eggshell powder, and powder-doped iron oxide-hydroxide modified eggshell achieving 100% Cr and 76.32% Ni removal, 95.93% Cd, and 100% Pb removal respectively; the combination of ideal parameters leads to better adsorption; and the regeneration capability of chicken eggshell leads to an environmentally viable method of disposing of waste adsorbents. The prospective quality of economical eggshell adsorbents for wastewater treatment applications can be evaluated with the help of this review.