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纳米银（silver nanoparticles, AgNPs）是三维空间中至少有一维处于1—100 nm的单质银颗粒[1]，其拥有高效、广谱的杀菌性能，因此广泛应用于医药、食品、化妆品、纺织品等领域[2]. 随着近年来AgNPs技术的不断发展[3]，越来越多的AgNPs产品在生产、使用和废弃过程中释放进入水环境[4-5]，并对水生生物产生毒害[6]，因此有必要深入了解其环境归宿及潜在危害.
AgNPs化学性质活泼，进入水环境后很容易在氧气（O2）和质子（H+）的作用下发生氧化溶解，释放出银离子（silver ions, Ag+）[7]. 由于银的氧化还原电位适中（Ψ（Ag+/Ag0=0.80 V）），自然界中Ag+也可被环境中普遍存在的天然有机质以及一些动物、植物和微生物等还原成零价的AgNPs[8-10]. 因此，水环境中AgNPs与Ag+会相互转化，呈高度动态性. 而由于形态不同，AgNPs和Ag+的毒性效应存在较大差异[11-12]. 例如，虽然AgNPs和Ag+都会对蚯蚓产生细胞毒性，但Ag+主要积聚在含胞质溶胶的部分，而AgNPs主要破坏细胞膜隔室[13]. 此外，AgNPs和Ag+的生物利用度及在生物体的富集过程也存在差异[14-15]. 因此，研究水环境中的AgNPs与Ag+的转化过程对评估AgNPs的生态风险具有重要意义.
溶解性有机物（dissolved organic matter, DOM） 是一类广泛存在于自然水体，由各种活性有机物（如腐殖酸（humic acid, HA）和富里酸（fluvic acid, FA）、蛋白质、多糖和胞外聚合物（extracellular polymeric substances, EPS））组成的非均质复合物[16]. DOM具有多种活性官能团，如硫醇（—SH）、醇/酚羟基（—OH）、醛、羰基、酮、醚基、羧基（—COOH）、胺和甲氧基等，因此其具有较强的氧化还原性，能够介导水体中重金属的迁移转化、毒性和生物利用度的改变[17-18].
现有研究表明，DOM是影响AgNPs和Ag+相互转化的重要因素之一[19-21]. 然而DOM对AgNPs/Ag+的氧化还原存在双面性[22-24]，既可氧化AgNPs释放Ag+，又可还原Ag+生成AgNPs，因此，在含有DOM的水环境中AgNPs/Ag+如何转化，环境风险会有多大，目前仍难以预测.
本文首先介绍了DOM促进/抑制AgNPs氧化溶解的机理，然后阐述了DOM还原Ag+形成AgNPs的机理，在此基础上总结了环境因素对DOM介导AgNPs与Ag+相互转化的影响. 最后提出了目前研究存在的不足，并为未来研究方向提供一定的建议.
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DOM可通过氢键、静电引力、疏水性作用、配体交换和离子架桥等方式吸附在无涂层AgNPs、聚乙烯吡咯烷酮包埋的AgNPs（PVP-AgNPs）或柠檬酸盐包埋的AgNPs（Cit-AgNPs）表面，改变其界面特性，进而影响AgNPs的溶解速率及溶解平衡[25-26]）. 多数研究表明，DOM在AgNPs表面的吸附抑制了AgNPs的氧化溶解和Ag+的释放，其机理可总结为以下3点：
（a）吸附在AgNPs表面的DOM会屏蔽AgNPs对光子的吸收，进而抑制AgNPs的氧化蚀刻及Ag+释放[27-28]. Zhang等[29]研究发现，由于光屏蔽效应，AgNPs在含有聚苯乙烯微塑料溶液中光氧化释放的Ag+浓度显著低于纯水环境.
（b）阻塞AgNPs表面活性位点并降低其与水体氧化剂（如O2、H2O2和·OH）及H+的反应性[30]，这是DOM抑制AgNPs氧化溶解的主要机理. Li等[31]发现，AgNPs的氧化与DOM在其表面的覆盖率呈反比，当全氟羧酸在Cit-AgNPs表面覆盖率为0、20%和50%时，Ag+释放量分别为35.5、31.4、18.8 µg·L−1.
（c）形成物理屏障限制AgNPs表面的Ag+扩散到溶液中，并将氧化释放的Ag+还原为新的AgNPs[32-33]. Fernando等[34]研究发现，HA介导下AgNPs在短时间内释放大量的Ag+，然而在较长时间后，溶液中Ag+会被还原成AgNPs，导致溶液中Ag+浓度降低.
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DOM也可促进AgNPs氧化释放Ag+. 如Ostermeyer、Zhang和Yang [35-37]等研究发现，添加600 mg·L−1的牛血清白蛋白、10 mg·L−1的HA和总有机碳含量（Total Organic Carbon, TOC）为10 mg·L−1 C的EPS后，溶液中Ag+含量分别是未加DOM时的2倍、2.5倍和3倍. DOM促进AgNPs氧化溶解的机理可总结为以下3点：
（a）DOM可通过官能团（如—COOH、—OH和—SH等）与AgNPs、Ag2O相互作用形成复合物，削弱Ag—Ag键和Ag—O键，从而促进Ag+的释放[38]. 而且，DOM还可以通过与吸附在AgNPs表面的Ag+络合，使反应（1）平衡右移，促进AgNPs氧化溶解[39].
Gondikas等[40]发现，半胱氨酸（cysteine, Cys）能通过—SH与AgNPs释放的Ag+配位结合，促进溶液中AgNPs的氧化溶解.
（b）DOM中含量较多的酸性官能团（如羧基和酚羟基等）在水环境中会电离释放H+，较高的H+浓度会促进AgNPs表面氧化层的溶解，释放Ag+[41]. Zhang等[36]研究发现，HA在溶液中的酸释放促进了AgNPs的氧化溶解. 虽然吸附在AgNPs表面的DOM一定程度上阻碍了AgNPs与O2和H+的相互作用，但吸附层是可渗透的，AgNPs依然可与O2和H+反应[31].
（c）DOM具有很强的光化学活性，其在光照下可生成H2O2、1O2和·OH等强氧化性的活性氧物质（reactive oxygen species, ROS），氧化AgNPs[42]. Tong等[43]证实了光照下聚苯乙烯微塑料产生的1O2和·OH可诱导AgNPs氧化溶解.
DOM介导下促进/抑制AgNPs氧化释放Ag+的机理可总结为图1.
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DOM/Ag浓度（物质的量）比会影响DOM对AgNPs表面活性位点的占用以及DOM-Ag配体的形成，进而影响AgNPs的氧化溶解. 一般来说，当DOM/Ag较高时，DOM会占据AgNPs更多反应活性位点，抑制AgNPs与O2和H+作用，Ag+释放显著减少[44]. 当系统中萨旺尼河腐殖酸（Suwannee River humic acid, SRHA）的TOC浓度由0增加至6.6 mg·L−1C时，2.78 mg·L−1的AgNPs释放Ag+浓度由1383 µg·L−1降低至339 µg·L−1[45]. 同样，Ag+的释放也随其他组分DOM（如多糖、蛋白质和胞外聚合物等）浓度的增加而显著降低[46-48].
而当DOM/Ag浓度比较低时，DOM占据AgNPs表面活性位点较少，其还可以通过鳌合Ag+而促进AgNPs氧化溶解[35]. Cáceres-Vélez等[49]研究发现，20 mg·L−1的HA促进了10 mg·L−1 AgNPs的溶解（DOM/Ag=2），而抑制了0.5、1、3 mg·L−1 AgNPs （DOM/Ag > 6）的溶解. Boehmler等[50]同样发现，当牛血清白蛋白浓度由0增加至2 nmol·L−1时，其可通过—SH鳌合Ag+使得粒径为10 nm的 Cit-AgNPs的溶解速率增加1.5倍.
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DOM是非均质的混合物，其种类复杂，性质多变，如元素含量、官能团和芳香性会存在差异. 因此不同DOM种类作用下，AgNPs的氧化溶解差异明显. Gunsolus等[51]研究发现，小马湖富里酸（Pony Lake fluvic acid, PLFA）对Cit-AgNPs氧化溶解的抑制作用强于同样浓度的SRHA和萨旺尼河富里酸（Suwannee River fluvic acid, SRFA），进一步探究发现DOM介导下Ag+释放量与DOM的S、N含量呈负相关. 高S、N元素的DOM对AgNPs/Ag+有很强的亲和性，因此会占据更多AgNPs表面活性位点进而抑制AgNPs氧化溶解[52]. 而对Ag+有强亲和力的官能团也可促进AgNPs的氧化溶解. Liu[39]和Gondikas[40]均研究发现，DOM可通过—SH络合Ag+从而促进AgNPs的氧化溶解. 芳香性较强的DOM不会占据太多AgNPs的活性位点，进而增强AgNPs的反应性. Pokhrel等[53]研究发现，溶液中较高芳香性的风化褐煤腐殖酸（Leonardite humic acid, LHA）作用下Ag+释放量是无LHA条件下的4—5倍.
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离子浓度会影响DOM的分子结构，改变DOM在AgNPs表面的吸附[54]，进而影响DOM介导的AgNPs氧化释放Ag+. 较低离子浓度下，DOM分子结构较膨胀，DOM在AgNPs表面的吸附会占据更多氧化位点，且许多环境阴离子（
${\rm{CO}}_3^{2-} $ 、${\rm{SO}}_4^{2-} $ 、Cl−、${\rm{PO}}_4^{3-} $ ）会与Ag+反应产生微溶或难溶性产物，覆盖在AgNPs表面，占据氧化位点，进一步抑制AgNPs的氧化溶解[55]. Zhao等[56]研究发现，在较低的离子浓度条件下（<0.01 mol·L−1 Cl−），随着DOM浓度的增加，溶液中Ag+释放量显著降低.在较高离子浓度环境中，DOM分子结构紧凑，吸附在AgNPs表面的DOM占据表面氧化位点较少，并且与释放的Ag+络合促进AgNPs氧化溶解[49, 57]. 高浓度的Cl−对AgNPs氧化溶解的影响最为显著[58]. 当Cl−/Ag <26750时，反应主要生成AgCl（s）沉淀；当Cl−/Ag ≥26750时，主要生成溶解性的配合物
${\rm{AgCl}}_x^{1-x}$ （如AgCl2−、${\rm{AgCl}}_3^{2-} $ 和${\rm{AgCl}}_4^{3-} $ ）[59]. DOM会与${\rm{AgCl}}_x^{1-} $ 络合，促进AgNPs的氧化溶解平衡右移[56]. Li等[57]研究发现Cit-AgNPs暴露的天然微咸水盐度越高，则水体DOM介导的Ag+释放量越大.阳离子（如Na+、Ca2+、Mg2+）会促使AgNPs聚集，减小比表面积，从而抑制AgNPs氧化释放Ag+[60]. 虽然有研究证明DOM抑制了AgNPs的聚集，但在二价阳离子（如Ca2+、Mg2+）作用下，吸附在AgNPs表面的DOM会通过络合Ca2+、Mg2+而桥连，发生更强烈的聚集，使Ag+释放量显著降低[61]. Huang等[62]研究发现， AgNPs在HA与Ca2+共同作用下氧化释放Ag+的浓度依次低于其在HA作用下、Ca2+作用下和纯水中的Ag+释放量.
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pH会影响AgNPs向Ag+的转化过程，且会改变DOM在水体中的分子结构，影响DOM的光化学反应，因此pH显然会影响DOM介导下AgNPs的氧化溶解. 在低pH条件下溶液中H+含量较高，会促进AgNPs的氧化溶解平衡（反应式1）右移[63]；并且低pH时DOM的分子结构较紧凑，占据AgNPs氧化位点较少，削弱了其对Ag+释放的抑制作用[64]；此外，相比碱性条件下，酸性溶液中DOM光生氧化性自由基显著增加[65]. 因此，低pH条件下促进了AgNPs的氧化溶解；反之，高pH条件下会抑制Ag+的释放.
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AgNPs具有很强的光吸收能力，短时间光照会促进AgNPs的光裂解及氧化蚀刻，迅速释放Ag+. Shi等[28]研究发现，光照下三磷酸腺苷包埋的AgNPs会在短时间内（≤1 h）迅速氧化，其Ag+释放量显著高于黑暗条件. 但长时间光照会破坏AgNPs表面涂层，使AgNPs失稳聚集，表面活性位点减少，进而氧化速率降低. Yu等[32]研究发现在短时间光照（≤ 12 h）下，Cit-AgNPs的平均溶解速率常数是长时间光照（70 h）下的4.5倍，且AgNPs发生明显聚集.
光照会促进DOM介导的AgNPs氧化或降低DOM对AgNPs氧化溶解的抑制作用. Rong等[66]发现，当SRFA的浓度为0、5、10 mg·L−1时，10 min光照可使Cit-AgNPs氧化率分别比黑暗条件下提高了3.4%、4.6%和6.0%. Yu等[67]研究发现，黑暗条件下，TOC浓度为5 mg·L−1 C的SRHA使1.02 mg·L−1的AgNPs在20 h后的Ag+释放量由830 µg·L−1降至100 µg·L−1；而20 h的光照后，Ag+释放量由738 µg·L−1降至225 µg·L−1.
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在黑暗/光照条件下，DOM均可介导Ag+还原成AgNPs，其还原机理可概括为以下3点，见图2
（1）黑暗条件下的自催化. 溶液中游离的Ag+通过沉积在Ag2O/AgNPs簇表面进而提高其氧化还原电位（游离状态：Ψ（Ag+/Ag0 = −1.8 V）；吸附在固态表面的Ag+：Ψ（Ag+/Ag0 = 0.7996 V），进而使DOM还原Ag+的反应在热力学上是可行的，如式（2，3）所示[68].
自催化过程可以被描述为以下几个步骤[69]：
DOM首先发生脱质子化，然后通过静电作用/络合作用与Ag+结合[70-71]，DOM-Ag复合物通过还原性官能团（如—COOH、—OH、—SH、醛基和酮基等）将e−转移给Ag+，生成AgNPs[39, 72].
（2）光照生成还原性自由基. 在光照条件下，DOM充当光吸收体，产生强还原性自由基（如
${\mathrm{e}}^{-}_{\mathrm{a}\mathrm{q}}$ 和${\mathrm{O}}_{2}^{•-}$ ）使Ag+被迅速还原[73]. Yin等[74]研究发现，HA在Xe灯照射下产生的${\mathrm{O}}_{2}^{•-}$ 介导了Ag+还原成AgNPs；而在溶液中添加超氧化物歧化酶（superoxidase dismutase, SOD）去除${\mathrm{O}}_{2}^{•-}$ 后，溶液中没有生成AgNPs，证实了${\mathrm{O}}_{2}^{•-}$ 在Ag+还原过程中的重要作用.（3）光照条件下配体-金属电荷转移（ligand-to-metal charge transfer, LMCT）. 当Ag+吸附到DOM表面后，光照促进DOM配体将e−转移至Ag+，进而生成AgNPs[75]. Hou等[76]研究发现，HA光还原Ag+生成AgNPs的速率随着溶液中Na+浓度的增加而显著降低，证实了Na+通过竞争HA表面金属离子结合位点进而抑制了HA通过LMCT还原Ag+.
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DOM介导的AgNPs形成速率可由以下式子表示[74]：
其中，r为AgNPs形成速率，单位h−1；[Ag+]为Ag+浓度，单位mg·L−1；[DOM]为DOM浓度，单位mg·L−1；A为AgNPs表面等离子体共振（Surface Plasmon Resonance, SPR）峰吸光度.
由于DOM介导的AgNPs/Ag+氧化还原是同时进行的，当DOM浓度一定时，需要足够的Ag+浓度才能实现AgNPs团簇的快速生长，这个浓度称为临界诱导浓度. 低于临界诱导浓度时，Ag+还原生成AgNPs不稳定，会马上被氧化，无法实现AgNPs团簇的生长[77]；而高于该浓度时，AgNPs生成速率与DOM浓度呈正相关. Xiong等[78]研究发现，初始浓度为30 µg·L−1的Ag+无法被EPS被还原成AgNPs；而Yin等[79]研究发现，初始浓度为0.2 mmol·L−1的Ag+会随DOM浓度的升高而加速转化为AgNPs.
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不同组分DOM的性质差异（比如对Ag+的吸附能力、芳香性、分子量和官能团）会影响DOM介导的Ag+还原成AgNPs. 高Ag+吸附性的DOM能更好地通过LMCT还原Ag+. Liu等[75]比较了吸附性很强的溶解性黑炭和吸附性较弱的SRHA对Ag+的光还原能力，发现溶解性黑炭介导的AgNPs生成速率显著高于SRHA. Nie等[80]研究发现，DOM的芳香成分会限制Ag+与DOM的还原性官能团结合. 低芳香性的泥炭HA 和泥炭FA可在24 h内介导Ag+还原成AgNPs；而同等浓度的高芳香性商用HA需要120 h才能介导Ag+还原成AgNPs. 由于低分子量的DOM光屏蔽能力较弱，Guo等[81]研究发现，分子量<3 kDa的DOM作用下Ag+光还原效率远高于>3 kDa DOM. 此外，DOM官能团的还原性会影响Ag+的还原. Nie等[80]研究发现，酚基比羰基具有更强的还原性，其利用NaBH4将羰基转化为酚基后，发现DOM介导的AgNPs生成速率和浓度均显著增加.
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环境共存离子会影响Ag+还原为AgNPs. 其中环境中常见的阴离子如
${\rm{CO}}_3^{2-} $ 、${\rm{SO}}_4^{2-} $ 、${\rm{PO}}_4^{3-} $ 、S2−和Cl−均可与Ag+反应生成微溶或不溶性银盐（如Ag2SO4 （Ksp=1.4×10−5）、Ag2CO3 （Ksp=8.5×10−12）、Ag3PO4 （Ksp=8.9×10−17）、Ag2S （Ksp=6×10−51）和AgCl （Ksp=1.8×10−10）等）. 微溶性的银盐可为AgNPs的生成提供成核位点，促进Ag+还原成AgNPs. 而难溶性的银盐（如Ag3PO4、Ag2S、AgCl）在黑暗条件下会迅速聚集沉淀，不利于AgNPs的生成. Dong等[82]研究发现相比于没有离子添加的条件下，${\rm{SO}}_4^{2-} $ 和${\rm{CO}}_3^{2-} $ 促进了HA还原Ag+，而${\rm{PO}}_4^{3-} $ 、S2−和Cl−与HA无法在黑暗条件下将Ag+还原成AgNPs.此外，一些具有光敏性的银盐（如AgCl）被认为是AgNPs的前体物质，在光照下其会激发出从价带跃迁到导带的电子e−，e−通过界面电子转移至AgCl表面的Ag+，从而生成AgNPs[83]. Cl−介导下Ag+还原可以由以下式子表示[84]：
离子浓度对Ag+转化成AgNPs影响很大. 当离子浓度较低时（如Cl−/Ag+ ≤5000），Cl−与Ag+结合主要生成AgCl，在光照下可被还原成AgNPs；而在高离子浓度下（Cl−/Ag+ > 1.8×105），则AgCl会转化为光活性较弱的
${\rm{AgCl}}_x^{1-x} $ ，难以生成AgNPs[84]. DOM可作为AgCl纳米晶体还原时的电子源，加速光照下AgNPs的生成并使AgNPs稳定. Xiong等[78]发现，光照下TOC浓度为12 mg·L−1 C的EPS在无Cl−溶液中生成的AgNPs吸光度仅为0.15，当Cl−浓度增加至10 mg·L−1，生成的AgNPs吸光度为0.3，而当Cl−浓度高到150 mg·L−1时，AgNPs吸光度又降低至0.15.环境中常见的阳离子（如Ca2+、Mg2+、Na+、Fe2+和Fe3+）也会影响环境中Ag+转化为AgNPs. 如在光照下DOM会与Fe2+/Fe3+形成氧化还原循环，催化DOM对Ag+的还原，如化学式（12—14）所示[85]. Yin等[86]研究表明，添加10 µmol·L−1的Fe2+/Fe3+可使DOM-Ag+溶液中还原生成的AgNPs浓度显著增加.
Ca2+、Mg2+和Na+等阳离子的存在会竞争DOM表面的吸附位点，抑制DOM对Ag+的吸附及还原，并且这些阳离子会压缩AgNPs表面的双电层，使其失稳聚集，导致溶液中AgNPs浓度降低[87]. Yin等[79]研究发现，溶液中 Ca2+浓度越高则DOM还原Ag+生成的AgNPs浓度越低，且AgNPs的粒径显著增大. 而其他贵重金属离子如（Au3+）对应的纳米粒子具有较高的内聚能，成核速率较快，因此这类金属会优先与DOM作用并生成相应的纳米粒子，吸附Ag+在其表面并提供成核位点，促进了Ag+转化为AgNPs[88].
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DOM还原Ag+受溶液pH影响显著. 随着pH的升高，DOM的氧化还原电位逐渐降低，这促进了Ag+转化为AgNPs[74]；而且高pH条件下DOM的酸性官能团去质子化，其表面电性更负，带正电的Ag+与带负电的DOM之间的强静电引力增强了Ag+与DOM络合，促进了光照条件DOM通过LMCT途径还原Ag+或黑暗条件下DOM通过给电子官能团还原Ag+[76].
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黑暗条件下DOM还原Ag+需要很高的活化能，而光照可以加快DOM介导的Ag+还原生成AgNPs速率[89]. 光照可作为AgNPs生成的催化剂，既会诱导DOM产生
${\mathrm{O}}_{2}^{•-}$ 和${\mathrm{e}}^{-}_{\mathrm{a}\mathrm{q}}$ ，又可激发AgNPs产生表面热电子，进而使Ag+快速转化为AgNPs[90]. Tan等[91]研究发现，模拟阳光照射下，90%以上的Ag+可在DOM介导下被还原成AgNPs，而在黑暗环境中Ag+还原率不到10%. Liu等[92]模拟了自然界的光暗交替环境，研究发现光照下HA可还原Ag+生成AgNPs，而黑暗中AgNPs则被氧化释放出Ag+. 但Dong等[82]的研究则表明，黑暗条件下DOM可还原Ag+生成AgNPs. 由此可见，环境中AgNPs与Ag+的相互转化是同时进行的，实验设置的差异导致Ag+还原速率与AgNPs氧化速率不一样，因此目前的研究结果常存在矛盾.鉴于研究DOM介导AgNPs/Ag+转化会得到DOM促进/抑制AgNPs氧化释放Ag+以及还原Ag+生成AgNPs 3种不同的结论，可能源于这些研究中设计的特定实验条件影响了DOM对
$ \mathrm{A}{\mathrm{g}}^{0}\rightleftarrows \mathrm{A}{\mathrm{g}}^{+} $ 的转化，如光照、pH、DOM种类、DOM/Ag物质的量比和电解质等. 现将这些研究中的反应条件展示于表1，以更好地看出先前研究中DOM介导AgNPs/Ag+不同转化的反应条件异同. -
目前研究表明DOM可通过占据氧化位点、还原Ag+、光衰弱来抑制AgNPs氧化，或释放H+、络合Ag+和光生氧化性ROS来促进Ag+释放，还可以通过自催化、光生还原性ROS和LMCT等途径将Ag+还原成AgNPs. 而环境因素（如DOM组分及浓度、AgNPs/Ag+浓度、离子、光照和pH等）会影响AgNPs/Ag+的转化过程，因此判断真实环境中DOM介导下AgNPs/Ag+的转化方向是比较困难的. 鉴于目前银基抗菌产品的市场在世界范围内进一步扩大，这将导致AgNPs与Ag+释放到生态环境中，并对生态系统和人体健康造成潜在危害，因此迫切需要更科学的理论基础去预测和评估其环境及健康风险.
目前DOM存在下AgNPs氧化/Ag+还原是AgNPs研究领域的热点之一，研究仍存在一些问题值得今后进一步研究：
（1）从表1可以看出，反应条件会显著影响DOM介导的AgNPs氧化及Ag+还原过程，区分和量化各种反应条件对这些过程的影响[26]，将有利于判定AgNPs与Ag+转化的进行.
（2）光照对DOM介导的纳米银/银离子转化过程具有重要的影响，光照既可促进纳米银转化为银离子，也可促进Ag+还原生成纳米银，因此难以预测在现实环境中光暗交替下纳米银/银离子转化，未来可加强这方面的研究.
（3）目前关于DOM与AgNPs相互作用的研究存在使用的DOM模型简单（常将HA/FA作为DOM模型）的问题[93]. 未来研究中应考虑研究其他DOM组分（如溶解性黑炭[94]和人工合成类DOM）与AgNPs/Ag+相互作用的效应及作用机理.
（4）AgNPs粒径、形貌和表面包被等对AgNPs氧化溶解的影响已研究得较为透彻，但目前尚不清楚在DOM存在的条件下AgNPs粒径、形貌和表面包被会怎样影响DOM与AgNPs的相互作用及AgNPs的氧化溶解. 今后可研究上述因素与DOM耦合作用下的AgNPs氧化溶解，并深入探究其机理.
溶解性有机质对纳米银/银离子转化的影响
Effect of dissolved organic matter on the migration transformation and toxicity of silver nanoparticles
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摘要: 纳米银（silver nanoparticles，AgNPs）的广泛应用导致其大量释放到水环境中，并在各种环境要素的作用下溶解产生银离子（silver ions，Ag+），对生态环境和人类健康造成了潜在威胁. 由于Ag+是AgNPs毒性的主要来源，探究水环境中AgNPs/Ag+相互转化有助于评估其环境风险. 溶解性有机物（dissolved organic matter，DOM）是地球上化学活性最强的物种之一，其在水环境中无处不在，既可介导AgNPs氧化释放Ag+，也能还原Ag+形成AgNPs. 由于反应条件的差异，先前研究DOM介导的AgNPs/Ag+相互转化常常得到相反的结论，因此难以预测具体环境中DOM介导的AgNPs/Ag+转化. 因此本文总结了DOM介导AgNPs氧化及Ag+还原的机理，并重点剖析不同环境因子对DOM介导AgNPs/Ag+氧化还原的影响. 本综述旨在为探究DOM介导AgNPs/Ag+氧化还原循环提供新的视野，并为AgNPs进入水环境后的归趋和风险预测提供科学依据.Abstract: The numerous applications of silver nanoparticles (AgNPs) leads to their spread in aquatic systems and the generation of silver ions (Ag+), which brings potential risks to environments and human health. Because Ag+ is the main toxicant source of AgNPs, the exploration of mutual transformations between AgNPs and Ag+ could help to evaluate the environmental risks in the process. Dissolved organic matters (DOMs) are ubiquitous on the earth and have high chemical activities. DOM could oxidize AgNPs to Ag+, and reduce Ag+ to AgNPs as well. Due to the different reaction conditions, previous studies always generate opposite conclusions in the DOM mediated transformations between AgNPs and Ag+, which causing the difficulty to predict the transformation in specific reactions. Here we summarized mechanisms of AgNPs oxidation and Ag+ reduction regulated by DOMs, and analyzed the environmental effects on DOM regulating the reactions between AgNPs and Ag+. The objective of review is to raise new perspectives to above mentioned processes, and provide references for the risk assessments while AgNPs entering into aquatic environments.
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Key words:
- dissolved organic matter /
- silver nanoparticles /
- silver ions /
- oxidation /
- reduction.
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图 1 DOM介导AgNPs转化为Ag+的机理
Figure 1. Mechanism of DOM mediated AgNPs transformation to Ag+
图 2 DOM 介导下Ag+转化为AgNPs的机理
Figure 2. Mechanism of DOM mediated Ag+ transformation into AgNPs
表 1 DOM介导下的AgNPs氧化或Ag+还原条件汇总
Table 1. Summary of AgNPs oxidation or Ag+ reduction conditions mediated by DOM
离子
IonAg初始浓度
Initial concentration of Ag speciesDOM种类及初始浓度
DOM species and initial concentration光照
Illumination statepH 时间
Time结果
Result参考文献
Reference0.08—16.67 mg·L−1 盐度天然水 5 mg·L−1 AgNPs 3.8—7.2 mg·L−1 C 天然水体DOM 黑暗 — 7 d 高盐度↑
低盐度↓Li et al., 2020[57] 0、0.01、0.1、0.3 mol·L−1 NaCl 1 mg·L−1 AgNPs 1.5—2 mg·L−1 C 天然水体DOM — 5.5 168 h ↑/↓ Zhao et al., 2021[56] — 20 mg·L−1 Ag+ 5—20 mg·L−1 C HA 光暗交替 7 96 h 光: ←；暗:↑ Liu et al., 2020[92] 7 mmol·L−1 NaHCO3
10 mmol·L−1 NaNO38 µmol·L−1 AgNPs 400 µmol·L−1 Cys — 7.5— 8.1 50 h ↑ Gondikas et al., 2012[40] 280 mg·L−1 CaCO3 10 mg·L−1 AgNPs 2—20 mg·L−1 LHA — 7 4 h ↑ Pokhrel et al., 2014[53] — 1.08 mg·L−1 AgNPs 0—10 mg·L−1 C EPS 132 W·m−2 Xe灯 7.1 72 h ↑ Yang et al., 2021[37] 柠檬酸盐缓冲液 1 mg·L−1 AgNPs 0—2 nmol·L−1 BSA — 6.5 4 h ↑ Boehmler et al., 2020[50] 硼酸盐、硝酸根 20 mg·L−1 AgNPs 20 mg·L−1 PS MPs 550 W·m−2 Xe灯/黑暗 5.5 72 h ↑ Tong et al.,2022[43] 硼酸盐、硝酸根 20 mg·L−1 AgNPs 20 mg·L−1 PS MPs 550 W·m−2 Xe灯/黑暗 8.5 24 h ↓ Zhang et al.,2021[29] 0.1 mmol·L−1 KH2PO4 500 µg·L−1 AgNPs 0.1—10 µmol·L−1 Cys — 7 72 h ↓ Afshinnia et al., 2018[52] 人工介质ASW38 1 µg·L−1 AgNPs 0—50 µmol·L−1 BSA 自然光 8 15 h ↓ Levak et al., 2017[47] — 1.02 mg·L−1 AgNPs 5 mg·L−1 C SRHA 550 W·m−2 Xe灯/黑暗 5—8.3 48 h ↓ Yu et al., 2014[67] 0.1 mol·L−1 KH2PO4 5 mg·L−1 AgNPs 10 mg·L−1 PLFA、SRHA、 SRFA — 7 5 h ↓ Gunsolus et al., 2015[51] 硼酸盐缓冲液 1—1000 µg·L−1 Ag+ 25 mg·L−1 SRHA 黑暗 6—9 2 d ← Dong et al., 2019[82] 0—150 mg·L−1 NaCl 5 mg·L−1 Ag+ 20、40 mg·L−1 EPS 荧光灯/黑暗 8 36 h ← Xiong et al., 2021[78] 0—10 µmol·L−1 Fe2+/Fe3+ 1 mmol·L−1 Ag+ 30 mg·L−1 DOM 550 W·m−2Xe灯 6.3 8 h ← Yin et al., 2017[86] 12.7 mg·L−1 NaCl 10 mg·L−1 Ag+ 50 mg·L−1 DBC/SRHA 50 W Xe灯 7.3 2 h ← Liu et al., 2021[75] 磷酸盐–硼酸盐缓冲液 1 mmol·L−1 Ag+ 15—100 mg·L−1 HA 黑暗 8 5 d ← Nie et al., 2020[80] — 0.2 mmol·L−1 Ag+ 20 mg·L−1 C EPS Xe灯/黑暗 7.6 16 h ← Zhang et al., 2016[90] 注：↑,↓分别表示促进和抑制Ag+释放；←表示促进Ag+还原生成AgNPs；-为文献未提及该因素；PS MPs代表聚苯乙烯微塑料；DBC代表溶解性黑炭；BSA代表牛血清白蛋白. Note:↑ and ↓ respectively promote and inhibit the release of Ag+;← means promoting Ag+ reduction to generate AgNPs; - is not mentioned in the literature; PS MPs stands for polystyrene microplastics; DBC stands for dissolved black carbon; BSA stands for bovine serum albumin. 
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