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厌氧消化(anaerobic digestion, AD)常常作为高负荷污水和固体废物处理的手段,经济高效,广泛应用于食品加工、制浆造纸、化学和石化等工业废水的处理[1]. 厌氧消化是一个多步骤的生物过程,由不同的微生物群体介导,依赖于系统中不同微生物的协调活动,微生物之间形成共生关系. 其具体过程可分为四个阶段:水解、酸化、产氢产乙酸和产甲烷,涉及的优势菌群分别为水解酸化菌与产甲烷菌[2]. 早期的研究发现水解酸化菌与产甲烷菌之间可通过氢气、甲酸等物质进行电子传递,这种间接种间电子传递(mediated interspecies electron transfer,MIET)的方式可实现能量的生成以及微生物的生长繁殖[2-4]. 进一步的研究发现两者之间存在更为高效的直接种间电子传递方式(direct interspecies electron transfer,DIET),微生物通过导电鞭毛、细胞色素c等直接完成种间电子传递[5-7]. 同时,有研究发现在厌氧消化体系中加入具有导电性能的非生物物质(如某些铁氧化物,石墨烯,生物炭等)可促进DIET,从而提高甲烷的生成速率和产量 [7-11]。这些研究聚焦于导电材料可以通过促进种间电子传递从而促进厌氧消化的宏观结果,但是对其促进种间电子传递的机理未做系统分析.
为了更深入理解厌氧消化过程中微生物种间的电子传递方式及其对厌氧消化过程的影响,本文从种间氢传递的机理入手,概述了种间电子传递如何帮助共生微生物克服其自身代谢反应的热力学障碍,随后介绍了具有导电性能的物质对DIET促进作用的研究现状,着重综述了铁氧化物对DIET促进作用和研究进展,分别从热力学、动力学以及理化反应的3个角度出发,归纳分析学者们对该作用发生机制的探究结果,指出当前研究存在的问题,并从分子水平分析DIET机制、DIET与异化铁还原过程的相互影响以及DIET工程应用等方面进行了展望,以期为此领域今后的研究提供参考.
铁氧化物促进微生物直接种间电子传递的机理及其研究现状
A review on enhancement of direct interspecies electron transfer induced by iron oxides and its mechanism
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摘要: 厌氧消化常作为高负荷污水和固体废物处理的手段,经济高效,有良好的应用前景。该过程由不同的微生物群体介导,微生物之间形成共生关系,从而克服代谢过程的热力学障碍. 在共生关系中,微生物种间电子传递过程极其重要,有机物氧化菌与产甲烷菌一般通过种间氢或甲酸传递进行种间间接电子传递. 随着研究进行,人们发现了电子传递效率更高的直接种间电子传递,可实现微生物之间直接电子交换,而不需要如氢气、甲酸等作为电子传递载体. 目前研究已表明具有导电性质的材料如某些碳材料以及铁氧化物能够促进直接种间电子传递. 为加深对种间电子传递的理解以期提高厌氧消化效率,本文陈述了厌氧消化种间氢传递和直接种间电子传递的机理以及非铁氧化物促进直接种间电子传递的研究现状,着重介绍了铁氧化物促进直接种间电子传递的研究进展,并分别从热力学、动力学、理化性质三个方面进行了分析,最后对铁氧化物促进直接种间电子传递的研究进行了展望.Abstract: Anaerobic digestion is often used as an economical and efficient method for high load sewage and solid waste treatment,which has a good application prospect. Anaerobic digestion is mediated by different microbial groups, in which symbiotic relationship is formed between microorganisms so as to overcome the thermodynamic obstacles of metabolic process. It is very important of interspecies electron transfer in the symbiosis. Indirect interspecies electron transfer between organic oxidizing bacteria and methanogens is generally carried out through interspecies hydrogen or formate transfer. With the progress of research, it has been found that direct interspecies electron transfer (DIET) with higher electron transfer efficiency can finish direct electron exchange between microorganisms without hydrogen and formate as electron transfer carriers. At present, studies have shown that materials with conductive properties, such as some carbon materials and iron oxides,can promote DIET. In order to get a further insight into the interspecies electron transfer to improve the efficiency of anaerobic digestion, this review describes the mechanism of interspecies hydrogen transfer and DIET. The research situation of DIET promoted by non-iron oxides has also been summarized. Besides, the research status of iron oxides promoting DIET has been emphasized, of which the promotion mechanisms studies has been analyzed in three aspects: thermodynamics, kinetics, physical and chemical properties. Finally, the research on iron oxides promoting DIET is prospected.
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表 1 丙酸和丁酸产氢产乙酸反应标准吉布斯自由能变(25 ℃)
Table 1. Standard Gibbs free-energy changes at 25 ℃ for reactions involved in propionate and butyrate oxidation in anaerobic systems
底物
Substrate反应
Reaction标准吉布斯自由能变/(kJ·mol−1)
Standard Gibbs free energy change丙酸 ${\rm{C} }{ {\rm{H} }_3}{\rm{C} }{ {\rm{H} }_2}{\rm{CO} }{ {\rm{O} }^ - }{\rm{ + 3} }{ {\rm{H} }_2}{\rm{O} } \to {\rm{C} }{ {\rm{H} }_3}{\rm{CO} }{ {\rm{O} }^ - }{\rm{ + HCO} }_3^ - {\rm{ + 3} }{ {\rm{H} }_2}{\rm{ + } }{ {\rm{H} }^ + }$ 76.1 $ {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{CO}}{{\rm{O}}^ - }{\rm{ + 2HCO}}_3^ - \to {\rm{C}}{{\rm{H}}_3}{\rm{CO}}{{\rm{O}}^ - }{\rm{ + 3HCO}}{{\rm{O}}^ - }{\rm{ + }}{{\rm{H}}^ + } $ 72.2 丁酸 $ {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{CO}}{{\rm{O}}^ - }{\rm{ + 2}}{{\rm{H}}_2}{\rm{O}} \to {\rm{2C}}{{\rm{H}}_3}{\rm{CO}}{{\rm{O}}^ - }{\rm{ + 2}}{{\rm{H}}_2}{\rm{ + }}{{\rm{H}}^ + } $ 48.1 $ {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{C}}{{\rm{H}}_2}{\rm{CO}}{{\rm{O}}^ - } + 2{\rm{HC}}{{\rm{O}}_3}^ - \to 2{\rm{C}}{{\rm{H}}_3}{\rm{CO}}{{\rm{O}}^ - } + 2{\rm{HCO}}{{\rm{O}}^ - } + {{\rm{H}}^ + } $ 45.5 表 2 部分底物甲烷化标准吉布斯自由能变(25 ℃)
Table 2. Standard Gibbs free-energy changes at 25 ℃ for some methanogenic reactions
底物
Substrate反应
Reaction标准吉布斯自由能变/(kJ·mol−1)
Standard Gibbs free energy change乙酸 ${\rm{C}}{{\rm{H}}_3}{\rm{CO}}{{\rm{O}}^ - } + {{\rm{H}}_2}{\rm{O}} \to {\rm{C}}{{\rm{H}}_4} + {\rm{HC}}{{\rm{O}}_3}^ -$ −31.0 氢气 $4{{\rm{H}}_2} + {\rm{HC}}{{\rm{O}}_3}^ - + {{\rm{H}}^ + } \to {\rm{C}}{{\rm{H}}_4} + 3{{\rm{H}}_2}{\rm{O}}$ −135.6 甲酸 $4{\rm{HCO}}{{\rm{O}}^ - } + {{\rm{H}}_2}{\rm{O}} + {{\rm{H}}^ + } \to {\rm{C}}{{\rm{H}}_4} + 3{\rm{HCO}}_3^ -$ −130.4 表 3 非铁氧化物促进厌氧消化DIET的研究
Table 3. Researches on direct interspecies electron transfer in anaerobic digestion
导电材料
Conductive materials底物
Substrates性能
Performance参考文献
References葡萄糖 甲烷生成速率和产量都有所提高 [16] 牛粪 甲烷生成速率提高,滞后时间缩短,挥发性脂肪酸(volatile fatty acid, VFA)和
丙酸浓度降低[17] 丁酸 滞后时间缩短,甲烷生成速率提高,避免丙酸积累引起的pH下降 [18] 生物炭 家禽粪便 此颗粒生物炭预装在高固体沼气池中90 d,可使甲烷总产量增加 69%,
每日甲烷峰值产量增加44%,与对照组相比,滞后时间缩短33%[19] 污泥 累计甲烷产量和最高甲烷生成速率分别提高了46.9%和181.6% [20] 橡果渣废料和
牛粪混合物橡果渣废料和牛粪比例为1:3时,沼气产量最大为580.9 mL·g−1;
总化学需氧量(total chemical oxygen demand, TCOD)下降了79.37%[21] 石油 粉状生物炭(< 5 μm)富集的微生物比颗粒状生物炭(0.5—1 mm)
富集的微生物多,二者均对甲烷产量有所提升[9] 碳布 垃圾焚烧
厂渗滤液碳布的加入使厌氧消化可以处理更高有机负荷的污水,
能够进行DIET的微生物大量富集在碳布表面[22] 合成废水 在高酸性和高氢分压下,DIET代替原有的IHT成为主要的工作机制,
同时代谢正常进行[23] 颗粒活性炭 人工乳制品废水 两相厌氧反应器产甲烷相中加入颗粒活性炭,醇类物质和
脂肪酸向甲烷的转化得到显着改善[24] 合成废水 底物加速降解,无活性炭的反应器甲烷累计产量第7 d达到峰值,
而加入活性炭的在4—5 d达到峰值[25] 合成废水 添加活性炭的反应器甲烷产量比没有加活性炭的反应器高1.8倍 [26] 污泥 颗粒活性炭从0到5 g,甲烷产量提高了17.4%,污泥消减率提高了6.1% [27] 污泥 颗粒活性炭反应器甲烷产量提高13.1% [28] 纳米石墨烯 合成废水 石墨烯(30 mg·L−1和120 mg·L−1)对甲烷的生成有明显的积极影响,
产量分别增加了17.0%和51.4%[29] 釜馏物 与对照组相比,纳米石墨烯反应组甲烷产量提升11%,消化滞后时间减少18.1% [30] 石墨烯 乙醇 高导电性石墨烯促进了甲烷的产量,提高了电子转移通量 [31] 活性炭 餐厨垃圾 15 g·L−1活性炭是提高甲烷产量的最佳剂量,中试实验最佳条件下
甲烷产量与对照组相比增加41%[32] 单壁碳纳米管 蔗糖 1000 mg·L-1单壁碳纳米管诱导了更快的底物利用率和甲烷产生速率 [33] 石墨毡 丙酸、丁酸 丙酸和丁酸降解期间,平均甲烷产量分别提高了19.1%和16.7%;
参与DIET的微生物丰度极大提高[34] 导电聚苯
胺纳米棒蔗糖 600 mg·L−1聚苯胺纳米棒可使甲烷产量翻倍 [35] 钨化合物(W, W2N, W18O49) 粪便尿液混合物 含钨化合物反应器沼气产量提升,消化时间减少 [36] 柚皮生物炭 乳牛粪 生物炭所在厌氧消化系统与对照相比,具有更高的累积沼气产量(525 mL·g−1 VS)和化学需氧量(chemical oxygen demand, COD)去除率(70.95%) [37] 石墨、生物炭 污泥 石墨反应器和生物炭反应器甲烷产量分别提高了38.3%和46.9%。在高H2分压下,
添加石墨和生物炭分别将甲烷产量提高了133.1%和63.7%[38] 生物炭、活性炭 葡萄糖、藻类水
热液化废水添加剂均缩短了厌氧消化滞后时间,提高了甲烷产量,
生物炭比活性炭更能提高甲烷产量[39] 石墨棒、碳布、
生物炭乙醇 COD去除率提高,均在93%以上 [10] 表 4 铁氧化物促进厌氧消化过程DIET的研究
Table 4. Study on direct interspecies electron transfer with addition of iron oxides
铁氧化物
Iron oxides底物
Substrates效应
Effects参考文献
References三氯乙烯 三氯乙烯脱氯速率提高1.5倍,其机理为磁铁促进了DIET [47] 高固体污泥 50 mg·g−1 TS磁铁矿明显减少了短链脂肪酸的积累,
并将甲烷生成速率提高了26.6%[48] 丙酸 丙酸的氧化和甲烷产量有显著的改善 [49] 乙酸、丙酸、丁酸 3种底物的降解率均有所提高 [50] 纳米Fe3O4 VFA(丁酸50%,丙酸25%,
乙酸25%)、合成废水磁铁的存在加速了丙酸降解,降低了电子传递的阻力 [11] 猪粪 75 mmol纳米磁铁矿将甲烷产量提升6%,日最大甲烷产量
提高47.8%,消化时间缩短20%以上[51] 稻草 磁铁的存在极大地促进了稻草的分解和甲烷的生成 [52] 油酸 甲烷产量有所增加,而且油酸浓度越高,甲烷产量越高
当COD为4 g·L−1时,甲烷产量增加114%[53] 煤气废水 纳米Fe3O4的加入降低了氧化还原电位以及生物毒性,
提高了污染物的去除效率[54] 乳清 文中所提到的磁铁回收与循环方法保证了长期不添加磁铁的产甲烷效率 [55] 丙酸 10 mg·L−1磁铁提高了约44%的甲烷生成速率,IHT以及DIET均起到作用 [56] 合成废水 最大甲烷生成速率提高了15.4%,滞后期缩短了13.9% [57] 合成废水 一个周期内最大甲烷产量提高了78.3%,
根据基因组分析磁铁的加入促进了DIET[58] 污泥 厌氧消化的效率提高,参加DIET的微生物大量富集 [59] 猪粪 最大甲烷产量增加16.1%,丙酸盐降解率得到改善 [60] Fe3O4 高浓度硫酸盐废水 磁铁存在的条件下不同反应器甲烷产量提高了3—10倍,污泥导电率提升了3倍 [61] 合成废水 磁铁矿的加入促进了VFA的降解和甲烷的生成 [62] 合成废水 长期补充磁铁矿可以有效减少VFA的积累,而且不同程度提高甲烷的产率 [63] 乳制品废水 更好的污泥沉降,电子传递;更高的电活性微生物丰度 [64] 猪粪 磁铁矿加速了有机物的水解,增加了Methanothrix , Methanospirillum的富集 [65] 污泥、餐厨垃圾 磁铁矿的加入增加了产甲烷性能,促进了DIET,
与此相关的微生物丰度有所提高[66] 污泥、餐厨垃圾 当污泥与餐厨垃圾的比例为1:0.5时,Fe3O4促进DIET,
VFA浓度提高,甲烷产量有所提升[67] 合成废水 磁铁矿的加入促进了厌氧硫氧化菌与产甲烷菌之间的DIET。 [68] 玉米秸秆+污泥 当Fe3O4为5 g·kg−1时,甲烷产量比对照组高60.47%;Geobacter与 Methanosarcina富集在Fe3O4表面,促进了DIET [69] 磁铁矿(Fe3O4>98%) 合成废水 除了降低氧化还原电位并促进VFA转化外,
磁铁矿的加入减少了核黄素和血红素c的合成[70] Fe3O4+ 乙醇 丙酸 Fe3O4和乙醇同时加入时,最大甲烷生成速率提高了81.4%,添加Fe3O4的
污泥电导率和电子转移系统活性分别增加了2.66倍和2.73倍[71] 纳米Fe2O3 甜菜制糖工业废水 750 mg·L−1纳米 Fe2O3 12 h后具有更高的COD去除率,甲烷生成率 [72] α-Fe 2O3 污泥、餐厨垃圾 缓冲了体系pH,增加了甲烷产量,尤其增加了丙酸、丁酸向乙酸的转化 [73] 赤泥(含45.46%
赤铁矿)污泥 甲烷产量增加了(35.52%± 2.64%),赤泥中的铁元素增加了电子转移效率 [74] 苯甲酸 赤铁矿和磁铁矿的加入,苯甲酸的降解率分别达到了81.8%和91.5% [75] 赤铁矿、
磁铁矿苯酚 二者均提高了甲烷产量以及苯酚的降解,同时产甲烷过程有较高的电子利用率 [76] 针铁矿 合成废水 增强了Syntrophomonadaceae的生长以及VFA的降解 [77] 表 5 铁氧化物促进厌氧消化反应动力学
Table 5. Kinetics of anaerobic digestion promoted by iron oxide
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