Abstract
Groundwater treatment sludge is a Fe/Mn-bearing waste that is mass produced in groundwater treatment plant. In this study, sludge was converted to a magnetic adsorbent (MA) by adding ascorbate. The sludge was weakly magnetised in the amorphous form with Fe and Mn contents of 28.8% and 8.1%, respectively. After hydrothermal treatment, Fe/Mn oxides in the sludge was recrystallised to siderite and rhodochrosite, with jacobsite as the intermediate in the presence of ascorbate. With an increment in ascorbate dosage, the obtained magnetic adsorbent had a significant increase in chromate adsorption but a decrease in magnetisation. When the Mascorbate/MFe molar ratio was 10, the produced MA-10 was a dumbbell-shaped nanorod with a length of 2-5 μm and a diameter of 0.5-1 μm. This MA-10 showed 183.2 mg/g of chromate adsorption capacity and 2.81 emu/g of magnetisation. The mechanism of chromate adsorption was surface coprecipitation of the generated Cr3+ and Fe3+/Mn4+ from redox reaction between chromate and siderite/rhodochrosite on MA-10, separately. This study demonstrated an efficient recycling route of waste sludge from groundwater treatment to produce MA for treating chromate-bearing wastewater.
Highlights
Chromate-containing wastewater, which needs to be effectively treated before discharging due to the high physiological toxicity of chromate to plants and animals, is widely produced in smelting and tannery factories [1, 2]
When the molar ratio was increased to 10, the Fe and Mn in product magnetic adsorbent (MA)-10, were 25.4 and 7.1 wt.% (Fig 1), apparently lower than those in the raw sludge and MA-1, which were assigned to the reductive dissolution of Fe/Mn at neutral condition (Fig 5A and 5B)
Fe and Mn were 28.8 and 8.1 wt.% in the sludge and were involved in the formation of jacobsite, providing the synthesized adsorbent with magnetic property. Such adsorbent was generated in four steps, namely, (1) the oxidation of ascorbic acid by dissolved oxygen to generate carbonate in the solution; (2) the reductive dissolution of Fe/Mn oxides by ascorbic acid to generate Fe2+ and Mn2+; (3) the reoxidization of Fe2+ by Mn oxides in the formation of MnFe2O4; (4) the carbonate accumulated in the solution and reacted with residual Fe2+ and Mn2+ to form siderite and rhodochrosite, respectively
Summary
Chromate-containing wastewater, which needs to be effectively treated before discharging due to the high physiological toxicity of chromate to plants and animals, is widely produced in smelting and tannery factories [1, 2]. The Chinese government has reduced the maximum discharging concentration of chromate to 0.1 mg/L [2]. Many strategies, such as chemical precipitation [3], ultrafiltration [4] and ion exchange and adsorption [2], have been applied to remove chromate from wastewater. Among these strategies, adsorption is considered as an economic and feasible method in treating chromate-containing wastewater. Industrial wastes, such as iron sludge from groundwater treatment [5], fly ash from coal combustion [6] and red mud from alumina refining [7], have been used as low-cost
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