Hydrothermal Conversion Of Fe/Mn-bearing Sludge To Adsorbent For Heavy Metals And Tetracycline Removal From Wastewater

Fe/Mn-bearing waste sludge is widely produced in the smelting and water treatment industries,and is usually used as a waste for safe and sanitary landfill.The utilization of Fe/Mn-bearing waste sludge to prepare adsorbents can not only save the cost of disposal and landfill,but also be used in waste water treatment,so as to achieve the purpose of treating waste with waste.In this paper,a simple hydrothermal method is used to convert three kinds of waste sludge?red mud,groundwater treatment sludge and Mn-bearing waste sludge?into magnetic adsorbent,polysulfide iron adsorbent and sulfur manganese adsorbent,applied to the typical water with heavy metal ions Zn²⁺and typical tetracycline antibiotics in water,focuses on the phase transformation principle and mechanism of action of iron-containing minerals and impurities in siliceous minerals in waste sludge.The main contents are as follows:Red mud is a typical iron-containing waste,which is rich in andradite,muscovite,hematite and cancrinite.Here,ascorbic acid is used as a reducing agent to convert red mud into a magnetic adsorbent by hydrothermal reduction.The results show that the main Fe-bearing mineral in red mud is andradite,which reacts with ascorbic acid under hydrothermal conditions,and the surface undergoes reductive dissolution,and then recrystallizes to produce magnetite and morimotoite.The hydrothermal temperature obviously promotes the dissolution of red mud,and the grain size of magnetite in the product increases significantly at high temperature.When the hydrothermal temperature reaches 200?,the saturation magnetization of the prepared magnetic adsorbent P-200 is 4.1 Am²/kg,which has a good magnetic response and can be quickly separated magnetically in water.It has a good adsorption effect on Zn²⁺in water,the maximum adsorption capacity is 89.6 mg/g,which is 8times of the original red mud.The adsorption of Zn²⁺by magnetic adsorbent P-200conforms to the Langmuir model,and cation exchange is the main adsorption mechanism.The main mineral phases of groundwater treatment sludge include ferrihydrite,quartz and boehmite,which are Fe-bearing precipitates formed in the production of Fe-bearing groundwater plants.Here,by adding Na₂S·9H₂O,a one-step hydrothermal method was used to convert the groundwater treatment sludge into an adsorbent containing erdite nanorods.The research results show that after hydrothermal treatment,the quartz and boehmite in the iron mud of the groundwater plant are dissolved and recrystallized into sodalite,while the ferrihydrite is transformed into erdite nanorods at 160? and hematite at 240?.When the hydrothermal temperature is 160?,the prepared erdite adsorbent SP160 is a nanorod-shaped particle with a diameter of 200 nm and a length of 2-5?m.SP160 is unstable under neutral conditions and automatically hydrolyzes to form Fe-bearing flocs,which has a good adsorption effect on oxytetracycline in water.The maximum adsorption capacity reached 2039.3 mg/g,which was three times that of the sample SP240 synthesized at240?.By adding 0.1 g SP160,nearly 100%of oxytetracycline and its derivatives in pharmaceutical wastewater can be removed.The main mechanism of removal is the spontaneous hydrolysis of erdite in SP160 to generate iron oxyhydroxide.The hydroxyl groups coordinate with oxytetracycline and its derivatives in the wastewater,thereby effectively removing oxytetracycline and its derivatives.After treatment of Mn-bearing groundwater,a Mn-bearing waste sludge will also be produced.Unlike Fe-bearing waste sludge,Mn-bearing waste sludge undergoes hydrothermal treatment to produce octahedral alabandite nanoparticles.Further,we focused on the effects of hydrothermal temperature and drying method on the formation of alabandite particles,and analyzed the hydrolysis products of alabandite and its adsorption of tetracycline in water.The results show that the product prepared by freeze drying in the range of 90-220? is octahedron alabandite.The vacuum drying and blast drying samples contained Mn?OH?₂,elemental sulfur and Mn₃O₄.With the temperature increasing from 90 to 220 ^oC,the diameter of prepared octahedron alabandite also increased from 2 to 5?m.The manganese sulfide crystals hydrolyze automatically under neutral conditions,and the resulting manganese-containing hydrate?MnOOH?exhibits a good adsorption effect on tetracycline hydrochloride in water.The samples prepared by freeze-drying showed the best adsorption effect of tetracycline,and the maximum saturated adsorption amount reached 2257.2 mg/g,which was better than the vacuum drying and blast drying alabandite samples.The above study clarified the principle of reduction and dissolution and recrystallization of andradite in red mud under hydrothermal conditions,and revealed the in-situ phase transformation of ferrihydrite in groundwater treatment sludge into erdite crystal and its automatic water decontamination mechanism.It also confirms the phenomenon that the thermal phase of Mn-bearing waste sludge turns into alabandite and its automatic hydrolysis in water,which provides a reference for the resource-based utilization of Fe/Mn-bearing waste.