Iron-based water treatment residuals as sorbent of heavy metals and metalloids

The ever-increasing water pollution caused by an increase in industrial activity in developing countries is a major worldwide problem. Heavy metal contamination is particularly dangerous because of their toxic and carcinogenic nature as well as harmful effects on human and animal health. Over the past decades, considerable efforts have been made to develop effective technologies for removing heavy metals from water. Adsorption seems to be the most promising out of the many methods. Conventional adsorbents used to remove heavy metals include activated carbon or clay minerals. However, due to the need for waste management, waste products have recently become very popular, especially industrial wastes containing iron and/or aluminum oxides. One of the possible sorbent are water treatment residuals (WTRs) which are generated during drinking water treatment process. The aim of this work was to examine the possibility of using residuals from deironing of underground water (G-WTRs) as effective sorbents of Cd (II), Pb(II), Zn(II), Cu(II), Cr(III), Cr(VI) P(V), and As(V) as a function of initial concentration, pH, temperature and time.

The G-WTRs were poorly crystalline and composed predominantly of ferrihydrite with minor calcite and quartz admixture. The main chemical components were iron (32%) and calcium (17%). Specific surface area was 144 m²/g with a total pore volume of 0.181 cm³/g. The proportion of micropores was 29%, mesopores occupied the greatest volume – 54%, while micropores the lowest volume – 17%.

Cation sorption efficiecy was almost 100%, in the case of anions it ranges between 50 – 100%. Sorption capacity increased with an increase in the initial pollutant concentration. Adsorption of the metal cations was higher with and increasing pH of the solution and the best results were obtained for pH 6.0 to 7.0. While anions were preferably sorbed in lower pH. Sorption was the efficient in the temperature range of 20-40 ℃. The greatest differences in the sorption efficiency were observed within the first 2 – 4 h. The possible sorption mechanism was chemisorption.

The results showed that G-WTRs can be effective and cheap sorbents of heavy metals and metalloids. However, further research including desorption process as well as the long-term stability of formed metal-G-WTRs complexes.

Acknowledgments: This work was financed by the National Science Centre, Poland Grant No. 2017/27/N/ST10/00713.