Preparation And Adsorption Characteristics Of Iron Modified Sludge Biochar

Wastewater containing chromium(Cr)and dyes are highly toxic and nondegradable,causing a significant threat to ecological environment and human health.Currently,there are many techniques for removing Cr and dyes from water,including adsorption,membrane separation and chemical precipitation.Adsorption has demonstrated great potential for the removal of heavy metals and dyes due to its environmental friendliness,efficiency and low cost.In addition,the disposal of residual sludge is one of the most difficult problems in wastewater treatment plants,but its pyrolysis into biochar can achieve the goal of "treating waste with waste".Based on this,in this study,the hydrophilic sludge biochar(HKBC),iron-modified sludge biochar(Fe-HKBC)and magnetic sludge biochar(Fe-S@HKBC)were prepared by residual sludge.The biochars were analyzed by the combination of spectroscopic and microscopic techniques.The adsorption characteristics and mechanisms of the biochars for Cr(Ⅵ),methylene blue(MB),and methyl orange(MO)were explored.The main results are as follows:(1)Activated sludge biochar(KBC)was prepared from residual sludge using a two-step activation method;and modified by acidic APS to obtain HKBC.The optimum conditions for the preparation of KBC were:carbonisation temperature of 600℃ and 1:2 mass ratio of SBC to KHCO3.BET and SEM-EDS indicated that HKBC had a specific surface area of 295.77 m2·g-1,which was 2.06 times larger than SBC,and had better hydrophilicity and polarity;FFT-IR and XPS analysis showed that HKBC was rich in oxygen-containing functional groups and amino functional groups.The optimum pH values for the adsorption of MB and Cr(Ⅵ)by HKBC were 10 and 1,respectively.The optimum dosage of HKBC was 4 g·L-1,and the adsorption equilibrium time were 90 min and 600 min,respectively.When the concentrations of MB and Cr(Ⅵ)were 500 and 300 mg·L-1,the adsorption amount could reach 120.26 and 37.21 mg·g-1,respectively.The adsorption processes both followed Pseudosecond-order and Langmuir models.Thermodynamic analysis showed that the adsorption were spontaneous and endothermic processes.The mechanisms of HKBC adsorption for MB were surface and pore adsorption,electrostatic attraction,π-πinteraction,hydrogen bond interaction and surface complexation.The mechanism of Cr(Ⅵ)adsorption is surface and pore adsorption,electrostatic force,reduction and ion exchange.(2)The HKBC was modified by FeSO4 to obtain Fe-HKBC.BET,SEM-EDS and XRD showed that there were a large number of mesoporous and microporous structures in Fe-HKBC,and the specific surface area was 280.68 m2·g-1.The surface was loaded with small particles and flake crystals formed by Fe0;FT-IR and XPS showed that there are a large number of alcohols,fatty ethers and iron oxide functional groups on the surface of Fe-HKBC.TGA and VSM analysis showed that Fe-HKBC had good thermal stability and superparamagnetism.The optimum pH value for Cr(Ⅵ)adsorption by Fe-HKBC was 1,with a suitable use of 4 g·L-1,and the presence of Na+and Ca2+ had an inhibitory effect on Cr(Ⅵ)adsorption.The adsorption of Cr(Ⅵ)by Fe-HKBC reached equilibrium at 600 min,with a removal rate of 95.22%.When the Cr(Ⅵ)is 300 mg·L-1,the adsorption amount reach 40.70 mg·g-1.The adsorption process fitted Pseudo-secondary and intraparticle diffusion kinetics and Langmuir isotherm model,indicating monolayer chemisorption is dominant,while intraparticle diffusion and pore channel adsorption were also present.The thermodynamic results suggest that adsorption was spontaneous and endothermic nature.Mechanisms of adsorption include pore adsorption,electrostatic gravitational interaction,reductive,surface complexation and ionic exchange.After 4 cycles of adsorption,Fe-HKBC still achieves 73.88%Cr(Ⅵ)removal.(3)Fe-S@HKBC was prepared by functional modification of HKBC with Fe2+and Fe3+.The properties of Fe-S@HKBC were analyzed by BET,FT-IR and XRD.The results showed that the specific surface area of Fe-S@HKBC was 283.96 m2·g-1,and many irregular flakes and needle-like crystals are formed on the surface of FeS@HKBC as α-FeOOH,Fe2(SO4)3,FeSO4 and FeS,as well as rich oxygen-containing functional groups,which provides sufficient active sites for adsorption.The optimum pH value for MO adsorption by Fe-S@HKBC was 1,and the suitable dosage of FeS@HKBC was 4 g·L-1.At 30 min,the removal rate of MO by Fe-S@HKBC reached more than 90%.In addition,Pseudo-second-order kinetics and Langmuir isotherm model provided a good fit for the adsorption process,and the maximum adsorption amount was 91.84 mg·g-1.Thermodynamic results indicated that the adsorption was a spontaneous and endothermic process.The mechanisms of Fe-S@HKBC for MO adsorption includes electrostatic attraction,redox,π-π interaction and hydrogen bonding.In addition,after 3 cycles of adsorption,the removal rate for MO can still reach77.31%,and VSM analysis showed that Fe-S@HKBC has good magnetic separation capacity with an applied magnetic force.Three kinds of sludge biochar obtained by different modification methods not only showed excellent removal ability,but also had efficient reusability.The two-step activation method can effectively improve the specific surface area and pore structure of biochar;acidic APS modification not only adds variety and number of organic groups to the HKBC surface,but also makes the biochar hydrophilic.After iron modification,Fe0 was loaded on the surface of Fe-HKBC,which had good synergistic effect and reduction effect in removal of Cr(Ⅵ)from water.After magnetic modification,the addition of iron ions to biochar can accelerate the adsorption efficiency of Fe-S@HKBC for anionic pollutants,and make Fe-S@HKBC have the ability to be easily recovered and separated.In summary,HKBC,Fe-HKBC and FeS@HKBC can be used as an economical and efficient adsorbent for the removal of dyes and heavy metal from water.At the same time,this study also provides a promising method for the resource utilization of sludge.