**Preface to "Various Adsorbents for Water Purification Processes"**

Water resources in the world are becoming increasingly scarce, and the demand for water is constantly increasing due to the growing population, rapid industrial development, or the resulting climate changes. On the other hand, the quality of available water is threatened not only by natural processes taking place, e.g., weathering of rocks, ocean evaporation, volcanic eruptions, etc., but primarily by anthropogenic human activity, which contributes significantly to the deterioration of water quality. Direct discharge of wastewater from municipal and industrial treatment plants without going through any treatment processes, or only after preliminary treatment, leads to many environmental problems, including deterioration of aerobic conditions for aquatic organisms, eutrophication of water bodies, and increase in pathogens and toxic compounds causing serious diseases [1]. Soil and water are most often contaminated with heavy metals, polycyclic aromatic hydrocarbons, chlorophenols, petroleum compounds, organic compounds, pesticides, pharmaceuticals, hormones, etc. Pb(II), Cd(II), Cr(III), Co(II), As(V), Zn(II), Hg(II), and Ni(II) are the most common heavy metals in wastewater. Like organic compounds, they can penetrate the human food chain. The non-biodegradability and bioaccumulative properties of these pollutants cause serious health problems in the human body, such as bone defects, elevated blood pressure, chronic asthma, and coughing [1–3]. Therefore, the treatment of wastewater before it is discharged into water bodies has become a necessity, and the development of effective treatment methods or the synthesis of new effective adsorbents capable of selective sorption of toxic substances is now widely and intensively studied [1]. Over the past few decades, physical, chemical, biological, as well as electrochemical and mixed methods have been used to remove organic and inorganic contaminants. Each of these has its advantages as well as disadvantages, so further research is needed to overcome them, and the topic of wastewater treatment is still very much in the news.

This Special Issue contains 12 articles [1–12] on wastewater treatment topics and is available online at: *Processes* | Special Issue: Various Adsorbents for Water Purification Processes (mdpi.com) (accessed on 26 May 2022). Among the papers, the vast majority are devoted to the removal of heavy metal ions, i.e., Ni(II) [2, 6], Pb(II) [3–5], Cu(II) [3, 8], Co(II) [7], As(V) [8], and Cd(II) [9] from synthetic and real solutions, and two papers on the removal of anabolic hormones, i.e., trenbolone [11] and β-estradiol [12] from water and wastewaters. Here, a comprehensive review concerning heavy metal ions removal by nano-adsorbents [1], as well as cost estimation of activated carbon production from waste nutshells by physical activation [10] could be found.

It was shown that the efficiency of the removal process of the above mentioned contaminants on synthetic ion exchangers [2], natural and synthetic aluminosilicates [3], zeolites [3], magnetic multiwall carbon nanotubes [4], leaves [5], date seeds powder [6], K2HPO4-pretreated Duckweed Lemna gibba [7], chitosan [8], fungi [9], imprinted polymer [10], and magnetic nano-Akaganeite [12] depends on many factors, including the chosen sorbent and its properties, as well as experimental conditions and the type and concentration of pollutants, but the proposed solutions are effective and interesting from a cognitive point of view. We invite you to take a detailed look at this work.

We are hoped that the papers collected in this Special Issue will inspire you for further studies on development of wastewater treatment techniques and may provide a basis for developing other creative approaches.

We congratulate and thank all the authors who made valuable contributions to this Special Issue. We also sincerely thank all the expert members and reviewers who spent their valuable time and make an effort in reviewed all the papers. The Guest Editors thank Amelia Qie for her dedicated and assistance in the development of this Special Issue. Thank you and best wishes for continued success.

### **References:**

1. Kumar, R.; Protima, R.; Erwan, R. Nanoadsorbants for the Removal of Heavy Metals from Contaminated Water: Current Scenario and Future Directions, Processes, 2021, 9(8), 1379. https://doi.org/10.3390/pr9081379.

2. Wołowicz, A.; Wawrzkiewicz, M. Screening of Ion Exchange Resins for Hazardous Ni(II) Removal from Aqueous Solutions: Kinetic and Equilibrium Batch Adsorption Method. Processes, 2021, 9(2), 285. https://doi.org/10.3390/ pr9020285.

3. Kozera-Sucharda B.; Gworek, B.; Kondzielski, I.; Chojnicki, J. The Comparison of the Efficacy of Natural and Synthetic Aluminosilicates, Including Zeolites, in Concurrent Elimination of Lead and Copper from Multi-Component Aqueous Solutions. Processes, 2021, 9(5), 812. https://doi.org/10.3390/ pr9050812.

4. Hanbali, G.; Jodeh, S.; Hamed, O.; Bol, R.; Khalaf, B.; Qdemat, A.; Samhan, S. Dagdag, O., Magnetic Multiwall Carbon Nanotube Decorated with Novel Functionalities: Synthesis and Application as Adsorbents for Lead Removal from Aqueous Medium, Processes, 2020, 8(8), 986; https://doi.org/10.3390/pr8080986.

5. Elkhaleefa, A.; Ali, I.H.; Brima, E.I.; Shigidi, I.; Elhag, A..B.; Karama, B. Evaluation of the Adsorption Efficiency on the Removal of Lead(II) Ions from Aqueous Solutions Using Azadirachta indica Leaves as an Adsorbent. Processes, 2021, 9(3), 559. https://doi.org/10.3390/pr9030559.

6. Elkhaleefa, A.; Ali, I.H.; Brima, E.I.; Elhag, A.B.; Karama, B., Efficient Removal of Ni(II) from Aqueous Solution by Date Seeds Powder Biosorbent: Adsorption Kinetics, Isotherm and Thermodynamics, Processes, 2020, 8(8), 1001; https://doi.org/10.3390/pr8081001.

7. Reyes-Ledezma,J.L.; Cristiani-Urbina, E.; Morales-Barrera, L. Biosorption of Co2+ Ions from Aqueous Solution by K2HPO4-Pretreated Duckweed Lemna gibba, Processes, 2020, 8(12), 1532. https://doi.org/10.3390/pr8121532.

8. An, B. Cu(II) and As(V) Adsorption Kinetic Characteristic of the Multifunctional Amino Groups in Chitosan, Processes, 2020, 8(9), 1194. https://doi.org/10.3390/pr8091194.

9. Kumar, R.; Sharma, P.; Umar, A.; Kumar, R.; Singh, N.; Joshi, P. K.; Alharthi, F.A.; Alghamdi, A.A.; Al-Zaqri, N. In Vitro Bioadsorption of Cd2+ Ions: Adsorption Isotherms, Mechanism, and an Insight to Mycoremediation, Processes, 2020, 8(9), 1085; https://doi.org/10.3390/pr8091085.

10. Leon, M.; Silva, J.; Carrasco, S.; Barrientos, N., Design, Cost Estimation and Sensitivity ´ Analysis for a Production Process of Activated Carbon from Waste Nutshells by Physical Activation, Processes, 2020, 8(8), 945; https://doi.org/10.3390/pr8080945.

11. Mpupa, A.; Dinc, M.; Mizaikoff, B.; Nomngongo, P.N. Exploration of a Molecularly Imprinted Polymer (MIPs) as an Adsorbent for the Enrichment of Trenbolone in Water. Processes, 2021, 9(2), 186. https://doi.org/10.3390/ pr9020186.

12. Mpupa, A.; Mizaikoff, B.; Nomngongo, P.N., Enhanced Adsorptive Removal of β-Estradiol from Aqueous and Wastewater Samples by Magnetic Nano-Akaganeite: Adsorption Isotherms, Kinetics, and Mechanism, Processes, 2020, 8(9), 1197. https://doi.org/10.3390/pr8091197.

Author Contributions: Writing—original draft preparation, A.W., M.W; writing—review and editing, A.W., M.W. All authors have read and agreed to the published version of the manuscript.

#### **Monika Wawrzkiewicz and Anna Wołowicz**

*Editors*
