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海藻酸钠对鸟粪石结晶的影响及机理研究

2017-10-13洪天求李如忠陈天虎

中国环境科学 2017年8期
关键词:晶体生长结合能鸟粪

韦 林,洪天求,李如忠,张 强,陈天虎*

海藻酸钠对鸟粪石结晶的影响及机理研究

韦 林1,洪天求1,李如忠1,张 强2,陈天虎1*

(1.合肥工业大学纳米矿物与环境材料实验室,资源与环境工程学院,安徽合肥 230009;2.合肥工业大学分析测试中心,安徽合肥 230009)

鸟粪石结晶沉淀法回收剩余污泥中磷工艺具有良好应用前景,然而污泥中的一些有机物,特别是胞外聚合物(EPS)将对鸟粪石结晶产生一定的影响.海藻酸钠(SA)在物化性质上接近于EPS,为较深入地探究此影响机理,以SA作为EPS的代用化合物采用恒组分、恒pH实验来考察EPS对鸟粪石结晶的影响,并借助红外光谱(FTIR)和X射线光电子能谱(XPS)分析来探讨SA与鸟粪石晶体之间的作用机理.结果表明,在SA浓度0~250mg/L范围内,随其浓度的增大鸟粪石结晶生长速率降低,鸟粪石晶体的尺寸有所减小,其原因是海藻酸阴离子吸附并掩蔽鸟粪石表面生长活性点上阻碍晶体生长.这种吸附作用是由鸟粪石表面Mg2+、磷酸羟基团(POH)分别与SA中COOH、C=O、COC键合共同引起的,而鸟粪石表面的NH4+并未参与反应.

鸟粪石;海藻酸钠;恒组分实验;晶体生长

随着磷排放标准不断提高,含磷废水一般需经过污水处理厂强化生物除磷工艺处理后方可达标排放.此时污水中90%以上的磷被转移到污泥中,污泥中磷含量可达6%~12%左右[1].鸟粪石是一种高品质的富磷矿,其P2O5含量高达50.3%[2];同时也是一种高效缓释复合肥料,常用于农业生产.因此,鸟粪石结晶沉淀法回收污泥中磷的潜力巨大,可缓解当前磷矿资源短缺问题,在污泥资源化方面也取得很好的经济效益[3].

对鸟粪石结晶沉淀法而言,要想高效地回收污泥中磷,首要是对污泥进行预处理使其中的磷转化为正磷酸盐形态充分地释放出来,随后投加镁盐进行鸟粪石结晶反应,最终通过沉淀分离回收磷[4].污泥预处理过程中不可避免地释放一定量的重金属和较大量的有机质[5].重金属对鸟粪石结晶影响已有较深入研究[6-7],但污泥中有机质对其影响的研究甚少.Uysal等[8]指出污泥中的重金属和多氯联苯对鸟粪石结晶影响不大,其原因之一是它们在污泥中含量较低.而污泥中大量的胞外聚合物[9]可能对鸟粪石结晶产生较大的影响.由于胞外聚合物成分复杂和结构的不确定性,其物化性质接近海藻酸钠(SA)[10],以SA作为EPS的代用化合物采用恒组分、恒pH实验来考察EPS对鸟粪石结晶的影响,并利用FTIR和XPS分析初步探讨鸟粪石晶体表面与SA之间的作用机理.

1 材料与方法

1.1 仪器及试剂

907型瑞士万通多通道自动电位滴定仪,配置3个自动加液单元,加液单元1为0.1mol/L NaOH溶液,加液单元2为0.096mol/L NH4H2PO4和0.096mol/L MgCl2×6H2O混合液,加液单元3为0.2mol/L NaCl和0.2mol/L NaOH混合液.实验所用试剂均为优级纯,所有储备液用超纯水配制.SA储备液配制是将一定量的海藻酸(Alrich)加入0.1mol/L NaOH溶液中,在室温下搅拌至完全溶解,再将pH调至9后备用[11].

1.2 鸟粪石结晶实验

鸟粪石晶体生长实验是采用恒组分、恒pH法在自动电位滴定仪上完成的[12-13].首先量取50mL 6mmol/L NH4H2PO4和50mL 6mmol/L MgCl2×6H2O与0.2mol/L NaCl混合液置于150mL滴定杯,并投加一定量的SA后密封置于磁力搅拌器进行搅拌.实验开始前,加液单元1用0.1mol/L NaOH将反应液pH快速调节至8.5.如反应式(1)所示,当结晶反应开始时,pH值随之下降.

Mg2++ NH4++ H2PO4ˉ= MgNH4PO4·6H2O + 2H+(1)

当溶液pH每下降0.001单位时,电位滴定仪自动从加液单元2、3同时向滴定杯中补加反应液,维持溶液恒定组分、pH值,从而保持整个反应过程中鸟粪石过饱和度不变、生长速率基本不变.反应过程中pH始终维持在8.5 ± 0.005,温度为25 ℃.反应2h自动停止,用0.45µm滤膜过滤,沉淀物去离子水清洗3次,置于40℃烘箱内干燥24h,放入干燥器内保存.每组实验均重复3次.软件将自动记录pH和加液单元的加液体积随时间变化.加液体积与时间成线性关系,鸟粪石晶体的生长速率(mol/min)可通过其斜率d/d由式(2)求出.

=tirant×d/d(2)

式中:titrant为加液单元2中Mg2+浓度(mol/L), d/d为加液体积与时间的拟合直线斜率.

利用场发式扫描电镜(SEM,SU8020,日本Hitachi公司),傅里叶变换红外光谱仪(NEXUS870型,美国NICOLET公司)和X射线光电子能谱(ESCALAB250Xi,美国Thermo公司)进行产物性质分析. XPS分析中矿物表面的结合能以284.8eV进行C1s校准,并采用XPSPEAKER41软件进行拟合分峰,用Shirley背景扣除, 峰型采用Lorentzian-Gaussian 函数(两者比率设定为20%).鸟粪石晶体和海藻酸钠的表面电势均采用马尔文zeta电位仪(Nano ZS90,英国Malvern公司)测定.

2 结果与讨论

2.1 SA对鸟粪石结晶的影响

SA对鸟粪石晶体生长速率及抑制率如图1所示,其生长速率随SA浓度在0~250mg/L范围内增大而降低,即从3.92×10-6mol/min降低到1.30×10-6mol/min,最大抑制率可高达60%以上.由鸟粪石SEM图(图2)可见,当溶液中无SA时,鸟粪石呈较粗的棒状结构.随着SA加入量提高,鸟粪石晶体呈明显的减小趋势.说明SA可以减小鸟粪石晶体尺寸和降低鸟粪石晶体生长速率,对鸟粪石结晶具有较强抑制作用.

基于一些文献关于有机物对矿物抑制作用机理[14-15],这种抑制作用可能有两方面原因加以解释.一方面,SA通过降低溶液的离子强度或与Mg2+生成络合物,从而导致溶液中鸟粪石饱和度的下降,鸟粪石晶体生长速率也相应地减小.另一方面,海藻酸盐可吸附、掩蔽在鸟粪石晶体表面生长活性点阻碍晶体生长,从而降低晶体生长速率.为检验鸟粪石结晶过程中溶液的Mg2+和SA是否发生络合作用,在相同实验条件(pH=8.5, 0.1mol/L NaCl和25℃)下,在自动电位滴定仪上利用0.1mol/L NaOH溶液分别对有、无添加3mmol/L MgCl2×6H2O时的100mL 200mg/L海藻酸溶液进行等体积(0.02mL)滴定.其结果如图3所示,两种滴定曲线偏移很小,说明SA与Mg2+络合作用很弱,可忽略不计.再者, SA是一种聚合物,其分子量一般在12000~80000之间,因而溶液中SA物质的量浓度远低于离子强度(0.1mol/L NaCl)几个数量级,说明SA加入并没有明显改变溶液离子强度.综上所述,这种抑制作用可推论为海藻酸阴离子吸附并掩蔽鸟粪石晶体表面活性生长点上,从而抑制鸟粪石晶体的生长.

通常聚合物在矿物表面上吸附作用可能是由静电作用力、氢键、化学键和疏水性等作用而引起的[16-17]. Zeta电位测定结果表明,在pH 8.5和离子强度0.1mol/L NaCl条件下,鸟粪石和SA表面电势均为负值,分别为(-11.2±0.5)mv和(-21.4±0.7)mV.两者之间存在静电排斥力,由此说明这种吸附作用是由静电作用力之外作用力引起的.为推测SA与鸟粪石晶体表面之间作用机理,分别对SA浓度为0mg/L、100mg/L和200mg/ L, pH=8.5时合成的鸟粪石(分别记作Str0、Str1和Str2)进行FTIR和XPS光谱分析.

2.2 FTIR分析

鸟粪石和SA的FTIR图谱如图4所示,无论溶液是否存在SA时,其合成物的红外光谱主要特征吸收峰的强度和位置变化不大,与先前文献中的鸟粪石的FTIR图谱基本吻合[18-19]. 572cm-1和1006cm-1处检测到显著的磷酸盐特征吸收峰;水分子之间和氨与水分之间的氢键吸收峰出现在762cm-1和887cm-1处;同时在3800cm-1与2200cm-1之间出现较宽的吸收峰为O-H和N-H的伸缩振动峰的重叠峰.说明SA仅吸附在鸟粪石表面且没有改变鸟粪石晶体结构. SA的FITR图谱中1178cm-1和1246cm-1是SA的COC对称和不对称伸缩振动峰[20-21].当溶液加入SA时,鸟粪石的FTIR图谱都在1168~1170cm-1和1236~1238cm-1范围出现新的吸收峰,可能是SA的COC与鸟粪石表面羟基形成氢键作用所致[22].1631cm-1和1429cm-1是SA的COOH反对称和对称伸缩振动峰[23],当溶液加入SA时,鸟粪石的FTIR图谱在1616~1608cm-1和1434~1440cm-1处也发生较小地偏移,其原因是SA的COOH与鸟粪石表面上Mg2+发生表面络合作用或与H2O和PO43-的水合羟基(POH)形成氢键作用[24].

2.3 XPS分析

鸟粪石XPS Mg1s窄扫描图谱如图5所示,当溶液pH 8.5且不含SA时,结合能为1304.4eV处出现一个左右对称的单峰.随着加入100mg/L SA和200mg/L SA时,峰形明显变宽,证明鸟粪石表面Mg和SA之间存在一定相互作用力.结合能为1303.5eV、1303.7eV和1304.4eV处均为MgO键[25-26],说明鸟粪石表面的Mg是以水合形式Mg(H2O)6存在[27].而结合能为1305.2eV和1305.0eV处鸟粪石表面的Mg是以非金属形态存在[28-30],推测鸟粪石表面Mg2+与SA的COOH形成了内层络合物.鸟粪石的N1sXPS窄扫描图谱如图6所示,无论溶液是否存在SA,其结合能均401.5eV处出现一个左右对称的单峰,其对应的基团为NH4+[31].随SA浓度增加到100mg/L和200mg/L时,其峰的位置和强度都无明显变化,说明鸟粪石晶体表面NH4+与SA基本不发生反应.

SA是一种水溶性阴离子聚合物,具有3种主要官能团即C=O、COC和COH,且易于与鸟粪石表面羟基形成氢键[32-33].为验证海藻酸阴离子与鸟粪石表面的POH或P=O基团是否存在氢键作用,需对鸟粪石表面P2p窄扫描谱拟合分峰,其结果如图7所示.P2p谱分出两个峰,位于134eV和133eV左右,对应的是POH和P=O[34-35].随SA浓度从0mg/L增加到200mg/L时,POH结合能也有所增强,分别为134.3eV、134.5eV和134.7eV,但结合能133.3eV处未发生明显化学位移,表明鸟粪石表面磷是以POH形式和海藻酸阴离子发生作用,而P=O并未参与反应.SA的C1s窄扫描图谱如图8所示,SA在284.8eV、286.5eV、288.0eV和289.2eV出现特征峰,对应的分别是污染峰(C-C,C-H)、COH或COC、C=O和O-C=O[36-38].当SA与鸟粪石作用时,鸟粪石的C1s谱除在284.8eV处出现C的污染峰外,其C=O和COC结合能随SA浓度增加均有所降低.C=O结合能由288.0ev降低到287.3eV、287.0eV;COC结合能从286.5eV减小到286.2eV、286.1eV.基于矿物与有机物氢键作用原理[39-40],可推测鸟粪石表面POH与SA中COC或C=O形成氢键作用.

3 结论

3.1 SA存在会减小鸟粪石晶体的尺寸,对鸟粪石晶体生长速率有明显地抑制作用,其最大抑制率可高达60%以上.

3.2 SA没有和溶液中Mg2+发生络合反应,也没有降低离子强度,对鸟粪石的过饱和度没有明显的影响;其抑制作用可推测为海藻酸阴离子吸附并掩蔽鸟粪石表面生长活性点上并阻碍鸟粪石晶体生长.

3.3 SA影响鸟粪石结晶的作用机理是一方面鸟粪石表面Mg2+与SA中COOH发生络合作用,另一方面鸟粪石表面POH与SA中C=O、COC发生氢键作用.

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Effect and mechanism of sodium alginate on struvite crystallization.

WEI Lin1, HONG Tian-qiu1, LI Ru-zhong1, ZHANG Qiang2, CHEN Tian-hu1*

(1.Laboratory for Nanomineralogy and Environmental Material, School of Resources & Environmental Engineering, Hefei University of Technology, Hefei 230009, China;2.Analysis and Measurement Center, Hefei University of Technology, Hefei 230009, China), 2017,37(8):2941~2946

The recovery of phosphorus from excess sludge by the crystallization of struvite has a good prospect. However, it may be influenced by some organic matter in excess sludge, especially by extracellular polymeric substances (EPS). To elucidate the effect mechanism of EPS, struvite crystallization from supersaturated solutions was investigated in the presence of sodium alginate (SA) presenting similar properties to EPS based on the characterization of Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. The results indicated that the growth rate and crystal size of struvite crystals significantly decreased with an increase of the SA concentration in a range of 0mg/L to 250mg/L. This phenomenon was assigned to the adsorption of SA on the growth site of struvite. Moreover, the adsorption was attributed to the interaction between Mg2+, POH in struvite crystals and the groups of COOH, C=O, COC in SA, whereas NH4+was not involved in the interaction.

struvite;sodium alginate;constant composition experiment;crystal growth

X703

A

1000-6923(2017)08-2941-06

韦 林(1980-),男,安徽庐江人,合肥工业大学博士研究生,主要从事水污染控制与资源化利用研究.发表论文10余篇.

2017-01-14

国家自然科学基金资助项目(41130206,51579061)

* 责任作者, 教授, chentianhu@hfut.edu.cn

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