肖辉毅,聂小保,万俊力,邓权庆,王奕睿,隆院男,蒋昌波

HAP诱导磷酸钙结晶回收低磷污水中的磷

肖辉毅,聂小保*,万俊力,邓权庆,王奕睿,隆院男,蒋昌波

(长沙理工大学水利与环境工程学院,洞庭湖水环境治理与生态修复湖南省重点实验室,湖南省环境保护河湖疏浚污染控制工程技术中心,湖南 长沙 410114)

为提高磷酸钙结晶产物回收价值和低磷适应性,以羟基磷酸钙(HAP)为晶种,诱导Ca-P结晶回收模拟二级出水(初始PO4-P浓度1.0mg/L)中的磷,对比了诱导结晶与均相结晶的磷回收效果,考察了Ca/P比和晶种投加量对磷回收的影响,并结合产物SEM、EDS、XRD和FTIR分析,探讨了HAP诱导Ca-P结晶机制.结果表明, HAP诱导Ca-P结晶在避免晶种材料对结晶产物纯度和品质影响的同时,还具有低磷适应性强和启动快速的优势; Ca-P结晶模式包括构晶离子在HAP表面的逐层结晶模式和在HAP颗粒空隙间的晶桥模式.实验条件下,模拟二级出水磷回收率可达80%以上,产物包括HAP及其前体物ACP(无定形态磷酸钙)和OCP(磷酸八钙). Ca-P结晶过程伴随发生CaCO3结晶,干扰磷结晶回收.研究成果为低磷污水磷回收率和回收产物品质的提升提供了依据.

HAP；低磷污水；磷回收；磷酸钙结晶；晶种

磷是有限的不可再生资源.据预测,到2050年我国将面临磷资源严重短缺[1],且磷是造成水环境污染的最主要污染物之一.从市政污水中回收磷,有望满足10%～15%的磷资源需求[2].

市政污水磷回收位点包括生物污泥及其灰渣[3-6]、污泥浓缩脱水液[7-9]、厌氧消化液[10-12]和二级出水[13-14].生物污泥中重金属、持久性有机污染物(POPs)和病原体浓度较高[15-17],不利于磷回收和回用;经焚烧后灰渣中磷含量可达50～100g/kg,但植物可利用性较低[9],重金属和铁/铝(污水混凝时加入)也会增加回收成本[9].污泥浓缩脱水液中腐殖质含量较高,会降低磷回收产品品质[18].厌氧消化液中磷浓度较高,且以PO4-P为主[19],便于结晶回收,但厌氧消化池和回收装置结垢明显,存在安全风险[20].据报道目前我国仅有不足50家市政污水厂厌氧消化装置仍在运行.

针对二级出水的磷回收,可避免或减轻上述问题,但也存在自身弊端,主要是水量大和磷浓度低,直接回收困难[9].为此,常采用离子交换进行预处理以提高磷浓度[14,21].但离子交换对磷的选择性不高,腐殖质、POPs和病原体会干扰离子交换装置运行,降低磷回收效率[22].目前鲜见针对二级出水的直接磷回收报道.

羟基磷酸钙(HAP)和磷酸铵镁(MAP)结晶可直接回收污废水中的磷[23-28],前者对低磷水质适应性更强[25].为进一步提高HAP结晶的低磷适应性,研究者以方解石[25-30]、改性方解石[27]、磷灰石(AP)[27]、石英砂[31]、牛骨粉[32-33]、转炉渣[34-35]、磁粉[36]、贝壳粉[37]等为晶种,借助晶种诱导作用降低结晶反应能垒,缩短结晶诱导期,从而在较低过饱和度下实现磷结晶回收.目前HAP诱导结晶原水磷浓度最低可至3.5mg/L[32].本文前期研究以方解石为晶种,通过增加晶种投加量(10g/L)和减小晶种粒径(45μm),对模拟二级出水(初始磷浓度1.0mg/L)获得80%以上磷回收[30].研究表明,诱导结晶效果主要决定于晶种表面对构晶离子的识别度[38],因此以HAP为晶种理论上可以获得最佳诱导效果;同时在结晶产品纯度、品质和回收价值方面具有优势.如以石英砂为晶种的HAP结晶产物,只能作为肥料回收,而以HAP为晶种,结晶产物有望作为回收价值更高的工业和生物原材料[39].

为此,本文以HAP为晶种、模拟二级出水为对象,开展低磷污水Ca-P结晶磷回收研究.考察HAP对Ca-P结晶的诱导作用,分析Ca/P物质的量比(Ca/P比)和晶种投加量对磷回收效果的影响,结合结晶产物SEM、XRD、EDS和FTIR分析,探讨HAP作为晶种的除磷机制.本文首次提出采用HAP作为晶种回收低磷污水中的磷,可为低磷污水磷回收率和回收产物品质的提升提供依据.

1 材料和方法

1.1 实验材料

CaCl2·2H2O、NaOH、HCl和KH2PO4均为分析纯(国药集团化学试剂有限公司).晶种粒径选择是诱导除磷的关键,减小粒径有助提升除磷效果,但粒径过小晶种容易流失导致出水浊度增加,为此采用100～150目HAP作为晶种,购自陕西飞米生物科技有限公司.溶液配制用水由Millipore Milli-Q Gradient水净化系统(Billerica, MA)制备,电阻率18.2MΩ·cm,pH=6.6～6.8.配制1.0g/L的磷溶液(以PO4-P计,下同)作为贮备液.NaOH和HCl贮备液浓度均为1mol/L.

原水采用自来水配制.自来水先经结晶去除钙硬度,再经离心分离(3000r/min)后,取离心液加入磷贮备液至预设初始磷浓度,最后采用NaOH和HCl贮备液调整pH值至9.0.水质指标如表1所示.

表1 原水水质指标

1.2 实验方法

1.2.1 HAP的诱导结晶效果 通过对比均相结晶(未投加HAP晶种)与诱导结晶(投加HAP)的磷回收率,考察HAP诱导结晶效果.对比实验采用六联搅拌仪(ZR4-6,深圳中润)进行.往烧杯中加入500mL原水(初始磷浓度分别为0.5、1.0、5.0、10.0、20.0和40.0mg/L),在快速搅拌条件下(300r/min)加入100目HAP,晶种浓度分别为0和10g/L.此后改为中速搅拌(150r/min),并投加Ca2+贮备液至Ca/P比为1.67,反应30min后再静置10min,取上清液测定磷浓度和pH值(过0.45 μm滤膜).

1.2.2 Ca/P比和晶种投加量的影响 往烧杯中加入500mL初始磷浓度1.0mg/L的原水,采用与1.2.1中相同的程序进行诱导结晶磷回收. Ca/P比取1.67、5、10、20、50和100;晶种投加量取0、0.5、1、2、5、10和20g/L.静置结束后, 取上清液测定磷浓度和pH值(过0.45 μm滤膜),再取一份上清液测定浊度(如有需要).

1.2.3 结晶产物表征 在流化床中(有效高度1.5m,内径2cm)投放10g/L晶种,流化床进水初始PO4-P浓度1.0mg/L、Ca/P比20、上升流速20m/h.稳定运行48h后,取晶种进行SEM、EDS、XRD和FTIR分析.

1.3 分析方法

pH值测定采用pH电极(雷磁PHSJ-3C,上海仪电科学);浊度测定采用浊度仪(2100P,美国哈希);磷浓度测定采用钼酸铵分光光度法,见《水和废水监测分析方法》(第四版)[40].

结晶产物40℃烘干后进行SEM与EDS分析(S4800,日本Hitachi)、XRD分析(D8-Advance,德国布鲁克)和FTIR分析(Nicolet iS50,美国赛默飞).将样品涂抹在电镜底座上,获得样品微观照片(SEM)和微观局部区域的元素分析数据(EDS),以射线能量的不同绘制X轴,能量强度为纵坐标绘制EDS分析图.对样品进行X射线衍射,得到样品的XRD谱图与标准图谱对照即可确定物质的组成相.根据红外光照射到样品上表面发生的电荷分布变化获得波数为横坐标,强度为纵坐标的红外光谱图,确定样品的官能团,对样品定性分析.

2 结果与讨论

2.1 HAP晶种的诱导效应

晶种的诱导效应是通过降低结晶体系固液界面自由能水平,达到类似催化结晶的效果,如减小结晶反应能垒和缩短结晶诱导期.

由图1(a)可知,初始PO4-P浓度小于10mg/L时,HAP晶种显著提高除磷率(<0.05)，且结晶速率快.在实验中观察到,当初始PO4-P为10mg/L时反应第1min去除率就达40%以上.而相同条件下Song等[27-28]以AP和方解石为晶种,去除40%以上PO4-P所需时间分别为2h和2d.这是因为磷结晶产物为HAP及其前体物,HAP作为晶种对其分子识别度显然最高.而且,有研究证实[31]其它材质晶种只有在表面逐渐被结晶产物覆盖后,诱导效应才开始发挥.因此,采用HAP作为晶种回收磷,在保证回收产物纯度同时,还有助于结晶回收装置的快速启动.

当初始PO4-P浓度超过10mg/L后,诱导结晶与均相结晶PO4-P回收率无显著差异(>0.05),说明均相结晶可顺利实现,无需外投晶种.Caddarao等[41]针对高磷废水(>500mg/L)设计的流化床均相结晶器(FBHC),无需外投晶种即可实现磷回收,且回收产品纯度高.Pahunang等[42]研究表明,初始PO4-P浓度100mg/L时均相结晶已能获得满意磷回收效果. Vasenko和Qu[19]研究证实,高磷废水(155mg/L)中晶种的加入,甚至会改变产物晶型,降低磷回收效率.

图1 HAP诱导结晶与均相结晶除磷效果对比

图1(b)表明均相与诱导结晶出水pH值无显著差异(>0.05),这与原水HCO3-碱度较高有关.虽然磷结晶回收率的提高会降低体系pH值,但由于CO32-～HCO3-缓冲体系的缓冲作用,降低并不显著.

2.2 Ca/P比对诱导结晶效果的影响

Ca/P比改变磷结晶体系SI,影响结晶速率和磷回收率.由图2a可知,初始PO4-P浓度1.0mg/L、Ca/P比1.67～100时,均相结晶磷回收率均不足10%,说明均相结晶动力SI不足.投加晶种HAP后,磷回收率显著增加,且随Ca/P比的增加而增加, Ca/P比超过20后,磷回收率达70%以上,上清液PO4-P浓度满足GB3838-2002的Ⅳ类标准[43].可见,通过提高Ca/P比增加SI,可有效提升诱导结晶速率和磷回收率.

图2(b)表明,尽管均相结晶时磷结晶率不足10%,但上清液pH值随Ca/P比明显降低,这可能是伴随的CaCO3结晶所致.原水中无机碳会发生 CaCO3结晶,引起pH值降低.相关研究[10,27]也证实Ca-P结晶往往伴随CaCO3结晶,两者竞争构晶离子(Ca2+)和晶种表面结晶位点,从而干扰磷回收.初始PO4-P浓度越低,HAP过饱和度越低,干扰可能越严重.考虑到HAP热力学溶度积常数(sp=10-58.52,25℃)要远小于CaCO3(sp,方解石= 10-8.48,25℃),控制结晶体系pH值至9以下,适度降低HAP和CaCO3的过饱和度(即结晶动力),有望减小无机碳对HAP结晶磷回收的干扰.

图2 Ca/P比对结晶除磷和上清液pH值的影响

初始PO4-P=1.0mg/L

2.3 晶种投加量对诱导结晶效果的影响

初始PO4-P浓度1.0mg/L、初始pH值8.0、Ca/P比20时,磷回收率和pH值随HAP投加量变化见图3.回收率和pH值变化均表明提高晶种投加量会促进磷的结晶回收.当HAP晶种投加量为20g/L时,出水磷浓度甚至满足GB3838-2002的Ⅲ类标准[43].

图3 晶种投加量对磷回收效果的影响

表2给出了相关研究中晶种使用情况.石英砂和转炉渣投加量最大[31,34-35],说明两者对Ca-P分子识别度最低.参考文献[34]中转炉渣投加量0.6g/L远小于[35]10g/L,与其粒径较小(15μm)相关.其余材质晶种投加量在1～30g/L间,与本研究接近.对方解石,初始PO4-P浓度越低,晶种粒径越小,可以提供更多结晶活性位点[44-45].但也有研究认为[25,29],结晶活性位点的增加会引起对构晶离子的竞争,导致晶种表面构晶离子浓度降低,反不利于低磷污水的磷结晶回收.本文磷回收率随晶种投加量持续升高,说明不存在构晶离子的竞争.分析认为,在低磷情况下,构晶离子不足以在整个晶种表面结晶,此时诱导结晶将优先在相邻晶种的空隙中进行,对晶种起到晶桥作用[46].晶桥模式产生的固液界面面积和界面自由能更小,更有利于低磷的磷结晶回收.

表2 相关研究中晶种的使用情况

磷结晶回收出需考虑出水浊度.图4中出水浊度与晶种投加量线性正相关,说明浊度主要由晶种引起.本文中由于初始PO4-P很低,磷的诱导结晶不会大规模生成难以沉降的微晶,浊度物质主要还是晶种中少量的细碎颗粒.可以预见,在实际生产中随着回收工艺的运行,出水浊度将逐渐降低.

图4 晶种投加量对上清液浊度的影响

初始PO4-P=1.0mg/L,Ca/P=20

2.4 结晶产物显微观测和表征分析

HAP晶种呈现规则的球形,表面光滑(图5a).相较于晶种,结晶产物粒径略有增加;同时晶体颗粒出现明显的两两团聚现象(图5b),包括大颗粒与小颗粒 (实线框所示)和粒径接近颗粒之间(虚线框所示)两种类型,分别对应表面逐层结晶和晶桥模式.

(a)HAP晶种 (b)结晶产物

初始PO4-P=1.0mg/L,Ca/P=20,晶种投加量10g/L

EDS分析表明,晶种主要元素为Ca、P和O,Ca/P比1.83(图6(a)),位于1.33～1.95之间,说明晶型以HAP为主.结晶反应后,晶种表面结晶产物中C含量明显增加,显然是磷酸钙结晶过程中伴随的CaCO3结晶所致.考虑CaCO3结晶,晶种表面结晶产物Ca/P比由1.83降低至1.41,表明结晶产物包括HAP及其前体物. 结晶产物XRD结果证实,晶种表面结晶产物包括HAP及其前体物无定型磷酸钙(ACP)和磷酸八钙(OCP),以及伴随的CaCO3结晶产物(图6(b)).

结晶产物FTIR谱图中(图6(c)),3570cm-1处特征峰对应HAP中O-H的伸缩振动峰[47],在1092, 1036,603和565处检测到PO43-特征吸收峰[10,48],说明产物中含HAP;1636cm-1附近特征峰为结晶水中O-H的伸缩振荡吸收峰[49],表明存在含结晶水的ACP.874cm-1处特征峰为CO32--特征吸收峰,进一步证实磷酸钙结晶过程伴随CaCO3结晶.

2.5 经济分析

仅以药耗对除磷/磷回收进行经济分析.某污水厂二级出水磷浓度约1mg/L,采用聚合氯化铁混凝除磷,平均药耗0.08元/m3.采用HAP诱导结晶回收磷,药耗来自氢氧化钠和氯化钙(晶种可循环再生,不计药耗);Ca/P=20时,两者投加量分别为10mg/L和72mg/L,市售价格分别为2000和700元/t,药耗0.07元/m3. HAP诱导磷结晶磷回收率以80%计,每m3污水可回收HAP 4.8g,市售工业HAP价格以140元/kg计,回收效益0.67元/m3,则污水厂采用HAP诱导结晶回收磷可收益0.6元/m3.采用转炉渣回收磷,市售转炉渣磷肥600元/t,在相同药剂投加量下,即便100%回收磷,效益也仅为0.038元/m3,药耗为0.032元/m3.可见,HAP诱导结晶磷回收经济优势明显.

初始PO4-P=1.0mg/L,Ca/P=20,晶种投加量10g/L

3 结论

3.1 采用HAP作为晶种,利用磷酸钙结晶回收低磷污水中的磷,避免了晶种材料本身对结晶产物的纯度和品质的影响.HAP对磷酸钙结晶产物的分子识别度高,有利于回收装置的快速启动.

3.2 HAP晶种表面的磷酸钙结晶模式,包括构晶离子在晶种表面逐层结晶模式和在晶种空隙间的晶桥模式.后者产生的固液界面面积和界面自由能更小,对低磷适应性更强.实验条件下,初始PO4-P浓度为1.0mg/L,磷回收率可达80%以上.

3.3 低磷浓度下,HAP诱导磷酸钙结晶的产物为HAP及其前体物ACP和OCP.碱度物质存在情况下,还会伴随CaCO3结晶, 干扰磷结晶回收.

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Phosphorous recovery from wastewater with low phosphorous concentration by means of HAP-seeded crystallization of calcium phosphate.

XIAO Hui-yi1,2,3, NIE Xiao-bao*, WAN Jun-li, DENG Quan-qing, WANG Yi-rui, LONG Yuan-nan, JIANG Chang-bo

(Key Laboratory of Dongting Lake Aquatic Eco-Environmental Control and Restoration of Hunan Province, Engineering and Technical Center of Hunan Provincial Environmental Protection for River-lake Dredging Pollution Control, School of Hydraulic & Environmental Engineering, Changsha University of Science & Technology, Changsha 410114, China)., 2022,42(4)：1681～1687

Phosphorous recovery from secondary treated effluent by crystallization of calcium phosphate (Ca-P), is one of effective strategies for phosphorous recovery from municipal wastewater. However, this strategy still needs further improvement, such as enhancing the quality of final products and improving the adaptability to low PO4-P concentration. In this paper, phosphorous in simulated secondary treated effluent with initial PO4-P concentration of 1.0mg/L was recovered by Ca-P crystallization using hydroxyapatite (HAP) as seeds. The recovery effects of induced crystallization were compared to that of homogeneous one, and the influence of both Ca/P mole ratio and dosage of seeds were investigated. Moreover, the induced crystal mechanism of Ca-P under low PO4-P concentration was studied based on phosphorous recovery effects and SEM, EDS, XRD and FTIR of crystal products. The results showed that the disturb of seed materials to the quality and purity of final products was minimized, since HAP was the major polymorph of final products. For simulated secondary treated effluent, HAP-induced Ca-P crystallization possessed high phosphorous recovery performance and rapid start-up. The induced crystal models of Ca-P crystallization included layer-by-layer crystallization on the surface of seeds, and crystal bridge model of which crystallization occurred within the space among seeds particles. Under experimental conditions, phosphorous recovery efficiency over 80% was obtained, with HAP and its precursors, ACP and OCP, the main polymorphs of final products. CaCO3crystallization was proved to take place simultaneously with Ca-P, which disturbs the recovery of phosphorous. The result provides new ideas for simultaneous improvement of recovery efficiency and the quality and purity of final products for low concentrated PO4-P wastewater.

hydroxyapatite (HAP)；low concentrated PO4-P wastewater；phosphorous recovery；calcium phosphate crystallization；seed

X703.1

A

1000-6923(2022)04-1681-07

肖辉毅(1998-),男,湖北鄂州人,长沙理工大学硕士研究生,主要从事污水污泥资源化利用方面研究.发表论文1篇.

2021-10-08

湖南省自然科学基金(2020JJ4609,2020JJ4631);长沙市科技计划项目(kq2005005);长沙理工大学水利与环境工程学院实践创新与创业能力提升计划项目(大型电站水库底栖动物的迁移运动特性与驱动机制)

*责任作者, 副教授, niexbcslg@163.com