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.尽管污水处理技术已取得长足进步，但废水中的重金属离子仍是全球性的重大环境污染难题。对这类重金属进行妥善处理，对防范人体健康风险至关重要，这也使得废水处理成为全球亟需解决的关键议题。由于传统处理系统无法完全去除水体中的有毒重金属，研究人员开始探索采用低成本材料的新型净化技术，以优化现有处理流程。各类低成本吸附剂（adsorbent）已被用于从废水及水溶液中提取重金属，原料涵盖农业废弃物、海鲜加工废弃物与食品加工废弃物。本综述旨在评估蛋壳用于废水处理中有毒重金属吸附的相关技术、应用优势、再生性能及未来发展前景，所针对的重金属主要包括铬（Cr）、镉（Cd）、铅（Pb）、砷（As）、镍（Ni）与汞（Hg）。本综述总结了影响最大吸附容量的各类关键参数，包括pH值、接触时间、反应温度、初始浓度以及吸附剂投加量。通过对比改性蛋壳吸附剂与天然蛋壳吸附剂的吸附动力学行为，本综述也为相关研究提供了更为全面的认知。本综述的核心结论如下：经化学或物理改性的蛋壳吸附剂具备更高的吸附容量，其中煅烧蛋壳、蛋壳粉以及掺杂羟基氧化铁改性的蛋壳粉分别可实现100%铬、76.32%镍、95.93%镉以及100%铅的去除率；优化参数组合可进一步提升吸附效果；而鸡蛋壳的再生性能，为废弃吸附剂的无害化处置提供了环境友好的解决方案。本综述可为评估经济型蛋壳吸附剂在废水处理领域的应用潜力提供参考依据。