Upcycling of Fe-bearing sludge: preparation of erdite-bearing particles for treating pharmaceutical manufacture wastewater

Abstract

Groundwater treatment sludge is a type of solid waste with 9.0-28.9% wt.% Fe content and is precipitated in large quantity from backwash wastewater in groundwater treatment. The sludge is mainly composed of fine particles containing Fe, Si and Al oxides, such as ferrihydrite, quartz and boehmite. The Fe oxides mostly originate from the oxidation of ferrous Fe in groundwater, whilst the silicate/aluminium compounds mainly originate from the broken quartz sand filter in the backwash step. In general, the sludge is firstly coagulated, dewatered by filter pressing and finally undergoes harmless solidification before it is sent to landfills. However, this process is costly (approximately US$66.1/t) and complicated. In this study, groundwater treatment sludge was effectively recycled to prepare novel erdite-bearing particles via a one-step hydrothermal method by adding only na 2 S•9H 2 O. After hydrothermal treatment, the quartz and boehmite of the sludge were dissolved and recrystallised to sodalite, whilst ferrihydrite was converted to an erdite nanorod at 160 °C and a hematite at 240 °C. SP160 was prepared as fine nanorod particles with 200 nm diameter and 2-5 μm length at a hydrothermal temperature of 160 °C. Nearly 100% OTC and its derivatives in pharmaceutical manufacture wastewater were removed by adding 0.1 g SP160. The major mechanism for the removal was the spontaneous hydrolysis of erdite in SP160 to generate Fe oxyhydroxide and use many hydroxyl groups for coordinating OTC and its derivatives. This study presents a novel method for the resource reutilisation of waste groundwater treatment sludge and reports efficient erdite-bearing particles for pharmaceutical manufacture wastewater treatment. Fe-bearing sludge is common in the metallurgical and chemical industries 1-3. Sludge contains 6.2-33.6% Fe, in the form of ferrihydrite, hematite, magnetite and andradite, and impurity minerals, such as quartz, muscovite, albite and boehmite 1,4. In China, tons of sludge are dumped and uncovered in massive piles 2. When rain occurs, Fe may leach and contaminate the soil and nearby surface water 5. Through strict environmental regulations, sludge is commonly dewatered and solidified before it is transferred to a safety landfill, but this process is costly and complicated 2,6-8. Resource reutilisation of sludge is an alternative strategy to reduce pollution and disposal cost. Many approaches have been developed to recycle sludge as a building material 9 , catalyst 10 and adsorbent 3,4,6,11. Amongst these approaches, the conversion of sludge to adsorbent is the most effective and simple, and the produced adsorbents are widely used in the adsorption of heavy metals 4,6,12 and cationic organics 1,3,11,13,14. For instance, groundwater treatment sludge and flocculent iron mud are rich in ferrihydrite and are directly applied to adsorb heavy metals, such as Cu and Zn 15. Ferrihydrite is effectively converted to maghemite and magnetite via hydrothermal route or calcination with the addition of a reductant, such as glycol 11 an...