Performance and mechanism of Cr(VI) removal by sludge-derived biochar loaded with nanoscale zero-valent iron
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摘要: 针对电镀、冶金、印染等行业产生的含铬废水所导致的环境污染难题,以城市污泥热解获得的污泥基生物炭(SB)为载体,制备了污泥基生物炭负载纳米零价铁(nZVI-SB)材料用于去除水中的Cr(VI),探究了铁炭质量比、初始pH值、投加量、温度等因素对去除Cr(VI)的影响。通过SEM-EDS、XRD和XPS等手段对nZVI-SB去除Cr(VI)的机制进行分析。结果表明:nZVI-SB对Cr(VI)废水具有较好的去除能力。在投加量0.5 g/L、初始pH=2、温度40℃条件下, Fe与SB质量比为1∶1的nZVI-SB(1∶1)对Cr(VI)吸附量最大为150.60 mg/g。Cr(VI)去除过程可通过Langmuir吸附等温式与准二级动力学方程进行拟合。nZVI-SB对Cr(VI)去除机制主要包括吸附、还原和共沉淀。本文表明污泥基生物炭与纳米零价铁可以协同发挥除Cr(VI)作用。Abstract: Chromium-containing wastewater was generated in electroplating, metallurgy, printing and dyeing industries, which caused environmental pollution. The sludge-derived biochar (SB) was obtained from the pyrolysis of municipal sludge, and then loaded with nanoscale zero-valent iron (nZVI) to prepare sludge-derived biochar loaded with nanoscale zero-valent iron (nZVI-SB) for the removal of Cr(VI) from water. The effect of the iron to carbon mass ratio, initial pH value, dosage and temperature on the removal of Cr(VI) were explored. SEM-EDS, XRD and XPS were used to characterize the mechanisms of Cr(VI) removal. The results show that nZVI-SB has a desirable removal capacity for Cr(VI). Under the conditions of dosage 0.5 g/L, pH=2 and 40℃, the maximum adsorption capacity of Cr(VI) is 150.60 mg/g by nZVI-SB(1∶1) with a mass ratio of 1∶1 between Fe and SB. The Cr(VI) removal process can be fitted by Langmuir adsorption isotherm and pseudo-second-order kinetic equations. The removal mechanisms of Cr(VI) mainly include adsorption, reduction and co-precipitation. The present study confirms SB and nZVI can synergically remove Cr(VI).
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Key words:
- sludge-derived biochar /
- nanoscale zero-valent iron /
- Cr(VI) /
- adsorption capacity /
- mechanism /
- reduction
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表 1 样品名称缩写
Table 1. Abbreviation name of samples
Sample Fe/wt% SB/wt% nZVI-SB(1∶4) 20.0 80.0 nZVI-SB(1∶2) 33.3 66.7 nZVI-SB(1∶1) 50.0 50.0 nZVI-SB(2∶1) 66.7 33.3 Notes: nZVI—Nanoscale zero-valent iron; SB—Sludge-based biochar. 表 2 污泥及污泥基生物炭(SB)消解液中重金属浓度
Table 2. Heavy metal concentrations in digestion solution of sludge and sludge-based biochar (SB)
(mg·L−1) Sample Zn Pb Cu Ba Cd Cr Sludge 5.28 0.34 2.37 7.32 0.06 0.14 SB 7.81 0.51 3.01 8.67 0.09 0.13 Specified value
in GB/T
5085.3—2007[18]100.00 5.00 100.00 100.00 1.00 15.00 表 3 SB和nZVI-SB的元素组成
Table 3. Elemental composition of SB and nZVI-SB
wt% Sample C N O Na Mg Al Si K Ca Cr Fe SB 40.48 8.46 34.94 0.19 0.39 3.26 6.43 0.88 3.05 0.33 1.59 nZVI-SB(1∶4) 36.63 7.56 33.05 0.22 0.48 0.63 0.76 0.28 0.30 0.22 19.87 nZVI-SB(1∶2) 30.41 6.41 27.55 0.18 0.36 0.51 0.62 0.23 0.24 0.18 33.31 nZVI-SB(1∶1) 23.92 3.11 21.45 0.14 0.27 0.28 0.35 0.19 0.21 0.16 49.92 nZVI-SB(2∶1) 16.48 1.76 14.45 0.06 0.13 0.11 0.16 0.08 0.08 0.06 66.63 表 4 nZVI-SB(1∶1)对Cr(VI)的吸附动力学参数
Table 4. Adsorption kinetic parameters of Cr(VI) adsorption by nZVI-SB(1∶1)
Intraparticle diffusion model qe/(mg·g−1) K/(min−1) R2 Kd/(mg·(m·min0.5)−1) C R2 Quasi-first order dynamics model 38.57 0.0113 0.855 5.858 22.731 0.971 Quasi-second-stage dynamics model 103.07 0.0005 0.999 0.038 98.551 0.946 Notes: qe—Equilibrium adsorption capacity; K—Adsorption rate constant; R2—Linear correlation coefficient; Kd—Particle diffusion constants; C—Constant. 表 5 nZVI-SB(1∶1)对Cr(VI)的吸附等温线拟合参数
Table 5. Adsorption isotherm fitting parameters of Cr(VI) by nZVI-SB(1∶1)
Temperature/℃ Langmuir Freundlich qm/(mg·g−1) KL R2 KF n R2 20 141.55 0.457 0.999 98.48 8.93 0.821 30 143.84 0.338 0.999 104.73 9.99 0.744 40 151.23 0.433 0.999 106.38 9.07 0.766 Notes: qm—Maximum adsorption capacity; KL—Adsorption equilibrium constant of the Langmuir model; KF—Adsorption equilibrium constant of Freundlich model; n—Constants related to the adsorption intensity. 表 6 nZVI-SB(1∶1)和其他吸附剂对Cr(VI)的吸附能力比较
Table 6. Comparison of the adsorption capacity of Cr(VI) by nZVI-SB(1∶1) and other adsorbents
Adsorbent pH Temperature/℃ Adsorption
capacity/(mg·g−1)Ref. Sludge biochar (500℃) 7 25 7.93 [13] Bentonite-supported nanoscale zero-valent iron (B-nZVI) 5 25 39.48 [23] Ficus carica biosorbent 3 30 19.68 [36] Magnetic nanoparticle-Phosphorene-Titanium nano tubes (MNP-PN-TNT) 9 25 35.00 [2] Nanoscale zero-valent iron grafted on acid-activated attapulgite (A-nZVI) 7 27 4.94 [31] HNO3 modified quinoa biochar 4 — 55.85 [37] ZnO modified hyacinth biochar — 25 43.48 [38] Halloysite nanotubes/ploy composites 2 25 855.66 [39] nZVI-SB(1∶1) 2 40 150.60 This study 表 7 nZVI-SB(1∶1)去除Cr(VI)前、去除Cr(VI)后的C1s、O1s、Fe2p和Cr2p XPS光谱的成分和相应的相对百分比
Table 7. Composition and relative percents of C1s, O1s, Fe2p and Cr2p XPS spectra before and after Cr(VI) removal by nZVI-SB(1∶1)
Components Relative percentage/% Binding energy/eV Before After Before After C1s C—C 58.99 59.78 284.64 284.69 C—O 26.24 24.00 286.06 286.27 C=O 14.77 16.22 288.54 288.72 O1s Fe—O 32.07 30.05 529.95 529.99 C—O 29.94 57.76 531.17 531.40 C=O 37.99 12.19 532.04 532.58 Fe2p Fe0 0.36 0.00 706.70 — Fe(II) 70.31 67.96 711.10/724.39 711.11/724.45 Fe(III) 29.33 32.04 714.41/728.15 714.44/728.55 Cr2p Cr(III) — 84.39 — 577.01/586.85 Cr(VI) — 15.61 — 580.40/590.28 -
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