Preparation And Adsorption/Catalytic Properties Of Fe/Si/Nitrogen-doped Biochar Derived From Waterworks Sludge

High organic content,difficult biodegradation and toxicity make the selection of dye harmless treatment technologies become the focus of researches.Considering the treatment effect and capital investment of single adsorption or catalytic oxidation,it is urgent to develop a green dual-function composite material with high activity,low cost and easy recovery.In recent years,advanced oxidation processes（AOPs）based on sulfate radical(SO₄^·-)has become a hot spot in the field of wastewater treatment.In order to solve the metal dissolution problem by metal-base activated persulfate,the combination of cheap carbon-based materials and transition metals is mostly used.As an inevitable carbon by-product in the process of water treatment,waterworks sludge is rich in iron oxides and silicates due to the use of flocculants,so it has become an urgent problem to find a way to maximize the utilization of waterworks sludge resources.In this paper,a kind of Fe/Si/Nitrogen-doped sludge-based biochar（NAl CS-Fe-Si）was prepared using waterworks sludge as raw material by the three-step process of nitrogen source introduction,Fe/Si leaching and magnetic particle loading,which combined inorganic components of sludge with carbonaceous matrix.Orthogonal experiments were designed to optimize the extraction and carbonization conditions of Fe/Si ion.Based on the element content in the sludge,the Fe/Si loading gradient was set to synthesize NAl CS-Fe-Si.Scanning electron microscopy（SEM）and nitrogen isothermal adsorption and desorption results showed that the preparation conditions had a great influence on the structure of sludge-based biochar.The increase of carbonization temperature,the introduction of nitrogen and the impregnation of acid and alkali could increase the number and specific surface area of sludge-based carbon pores,which contributed to the increase of adsorption sites.The introduction of iron and silicon indicated the homogeneity of the structure of sludge-based carbon pores,which was convenient for the subsequent stable catalysis of MB.The adsorption capacity of NAl CS-Fe-Si was investigated with MB solution of definite concentration as the treatment object.Results showed that the alkaline environment was conducive to the rapid removal of MB,and the adsorption of MB by NAl CS-Fe-Si at p H=9 followed the pseudo-second-order kinetic model and Langmuir model.According to the characterization of adsorbents before and after the reaction,the removal mechanism of MB mainly includes electrostatic attraction,hydrogen bonding andπ-πinteraction force between NAl CS-Fe-Si and MB.The results illustrated that NAl CS-Fe-Si still had about 70%MB removal ability after alternate desorption by ethanol and water.The degradation of MB by NAl CS-Fe-Si was investigated using peroxymonosulfate（PMS）as activator.Under the conditions of 0.1 g/L catalyst dosage,0.4 m M PMS concentration and default p H（5.85）,98%of 100 mg/L MB solution could be degraded within 20 min.The influence of coexisting substances in water showed that humic acid could promote the degradation of MB very quickly,while Cl^-had the strongest inhibitory effect on the activity of generating free radicals.By electron paramagnetic resonance（EPR）and quenching experiments,the degradation mechanism of MB was speculated as follows:SO₄^·-generated by Fe²⁺,Fe³⁺,C-OH and-COOH activation of PMS and ¹O₂ generated by NAl CS-Fe-Si as the medium of electron transferred from MB to PMS were the main active substances in the complex,and·OH also participates in the degradation of MB.