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随着人类社会快速发展,环境与资源问题日益凸显。目前普遍认为,人为因素造成的磷污染是水体富营养化的主要成因之一。水体富营养化将引起藻类快速繁殖,降低水体溶解氧,最终导致水生动植物死亡、生态系统失衡[1]。《2021年中国生态环境状况公报》显示,我国110个重要湖泊中,富营养状态湖泊占比高达27.3%[2]。湖泊富营养化已然成为我国亟待解决的重大环境问题之一。治理水体磷污染迫在眉睫。
近年来,国内外学者就水体磷污染控制开展了诸多研究,所采取的方法包括吸附法、化学沉淀法以及生物法等。其中,吸附法因其操作简单、处理迅速和效果稳定等优势受到关注。生物炭是近些年新兴的吸附材料,其由生物质热解碳化而得,具有较大的比表面积、发达的孔隙结构、丰富的官能团、强阳离子交换能力和低廉的价格等优势[3]。用农林废弃物等废弃生物质制备生物炭并用于去除废水中的磷,是当前的研究热点,具有固废资源化、固碳减排及磷资源回收等显著环境效益的优点。然而,大部分生物炭表面带负电,对磷酸盐的吸附效果不佳[4]。因此,需对生物炭实施改性,通过改变表面荷电性、增加吸附位点等方式提升其吸附磷的能力。常见的改性方法是在生物炭表面负载Ca[5]、Mg[6]、Ce[7]、Al[8]和La[9]等金属。在这些金属中,Mg具有在地壳中丰度高、对磷酸盐亲和力强的特点,非常适合用于生物炭改性[10]。在这方面,国内外已有较多相关研究。例如,研究者利用花生壳[11]、烟草秸秆[12]、槐木屑[13]、芦苇[14]和桉木茎片[15]等废弃生物质制备生物炭,然后用高浓度的Mg化学试剂对生物炭施以改性,并将改性生物炭用于废水除磷。这些研究表明,Mg改性生物炭对废水中磷的去除效果较好,具有一定实际工程应用的潜力。
虽然Mg改性生物炭除磷的研究取得了一定进展,但现有研究仍存在以下不足:以高浓度Mg化学试剂为改性剂增加了Mg改性生物炭的制备成本;Mg改性生物炭多呈粉末状,大量投加时易扬尘,且存在可操作性差、固液分离难的问题;多数研究集中于静态吸附实验,Mg改性生物炭的动态吸附研究相对较少[16-17]。可见,缺乏兼具经济性和实用性的Mg改性生物炭依然是该领域的瓶颈问题。开发低成本、具备一定尺寸且易于实际应用的Mg改性生物炭,并开展其在吸附柱中的动态吸附研究势在必行,是应对上述问题的有效策略之一。在前期研究中,ZHANG等[18]利用天然海水代替高浓度Mg化学试剂对生物炭粉末进行改性,发现不但改性效果好(改性后生物炭吸附容量提升了127倍),且改性过程几乎不增加生物炭的制造成本。这为制备兼具经济性和实用性的Mg改性生物炭提供了重要参考。据悉,目前尚无以海水为Mg源制备兼具经济性和实用性的Mg改性生物炭的研究报道。因此,本研究以天然海水为廉价Mg源、以生物炭颗粒(biochar granule,BC-g)为基材,制备低成本海水改性生物炭颗粒(seawater-modified biochar granule,SBC-g);利用表征技术考察SBC-g的物理化学性质及其吸附磷酸盐机理;在吸附柱中拟开展动态吸附实验,考察初始质量浓度、柱高、流量等因素对除磷效果的影响;并进行SBC-g制备成本的经济分析。本研究结果有望突破Mg改性生物炭除磷领域中存在的瓶颈问题,能够为兼具经济性和实用性Mg改性生物炭的制备及其在水体磷污染治理领域的实际应用提供参考。
低成本海水改性生物炭颗粒的制备及其对水中磷的动态吸附性能
Fabrication of low-cost seawater-modified biochar granule and its dynamic adsorption performance for phosphate in aqueous solution
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摘要: 针对生物炭除磷领域中缺乏兼具经济性和实用性的Mg改性生物炭的问题,以海水为廉价Mg源,制备了海水改性生物炭颗粒(SBC-g),探究了其物理化学特性和吸附磷酸盐机理,考察了柱高、流量和初始质量浓度对SBC-g动态吸附磷酸盐的影响及对含磷养殖尾水的处理效果,并对SBC-g进行了经济性分析。结果表明,改性后SBC-g表面负载的Mg(OH)2纳米片可增加吸附的活性位点,增大了介孔的孔径和孔容,改变了表面电荷性质,从而提高了其对磷酸盐的吸附容量。在一定范围内,柱高的增加或流量和初始质量浓度的降低均可延长穿透时间。Thomas模型对穿透曲线拟合良好(R2 > 0.919),可以较为准确地反映动态吸附过程。SBC-g吸附柱对养殖尾水具有良好的除磷效果,在最佳条件下吸附柱的穿透时间为589 min,磷饱和吸附量为1 051 mg·kg−1。SBC-g的生产成本约为2.65 元·kg−1,和其他除磷吸附剂相比具有较大的价格优势,兼具经济性和实用性。该研究结果可为Mg改性生物炭的制备及其在水体磷污染治理领域的实际应用提供参考。Abstract: There is a lack of low-cost and easy-operation Mg-modified biochar in the field of phosphorus removal by biochar, and thus seawater-modified biochar granule (SBC-g) was prepared using seawater as a cheap Mg source. The physicochemical properties of SBC-g and mechanism of phosphate adsorption were investigated. The effects of column height, flow rate and initial concentration on the dynamic adsorption of phosphate by SBC-g, as well as the removal performance of phosphorus from aquaculture tail water, were studied. Finally, the economic analysis of SBC-g was carried out. The results showed that the Mg(OH)2 nanosheets loading on the surface of SBC-g after modification increased the active sites, enlarged the mesopore size and volume, and changed the surface charge properties, thereby increasing the adsorption capacity for phosphate. Within a certain range, the increase of column height or the decrease of flow rate and initial concentration could prolong the breakthrough time. The Thomas model fitted well with the breakthrough curve (R2 > 0.919), which could accurately reflect the dynamic adsorption process. The SBC-g-loaded adsorption column exhibited a good performance on phosphorus removal from the aquaculture tail water. Under the optimal conditions, the breakthrough time was 589 min, and the saturated adsorption capacity of phosphorus reached 1 051 mg·kg−1. The production cost of SBC-g was about 2.65 ¥·kg−1, which had a great price advantage against other phosphorus removal adsorbents, and thus SBC-g could be regarded as a low-cost and easy-operation adsorbent. The results of this study can provide a reference for the preparation of low-cost and easy-operation Mg-modified biochar and its practical application in the field of phosphorus pollution control for waterbodies.
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Key words:
- biochar /
- seawater /
- Mg-modified /
- phosphate adsorption /
- adsorption column
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表 1 海水和养殖尾水水质参数
Table 1. Water quality parameters of seawater and aquaculture tail water
海水 pH PO43−
/(mg·L−1)K+
/(mg·L−1)Na+
/(mg·L−1)Ca2+
/(mg·L−1)Mg2+
/(mg·L−1)8.4 0.05 349.2 8 561.00 353.7 1 027.83 养殖尾水 pH 硝氮/(mg·L−1) 总氮/(mg·L−1) 活性磷/(mg·L−1) 总磷/(mg·L−1) 总有机碳/(mg·L−1) 7.05 5.35 6.72 2.02 2.15 6.47 表 2 改性前后生物炭比表面积、介孔孔径及孔容
Table 2. Specific surface area, mesopore size and volume of biochar before and after modification
生物炭 比表面积/(m2·g−1) 平均孔径/nm 总孔容/(cm3·g−1) BC-g 350.15 3.28 0.03 SBC-g 55.24 16.57 0.23 表 3 不同条件下吸附柱参数及Thomas模型的拟合参数
Table 3. Adsorption column parameters and fitting parameters of Thomas model at different conditions
C0/(mg·L−1) Q/(mL·min−1) H/cm m/g ta/min tb/min qae/(mg·kg−1) r/% KTH/(L·(min·mg)−1) q/(mg·kg−1) R2 2.00 1.5 3.5 1.3 141 1.83 244 75.00 3.60×10−3 531 0.922 2.00 1.5 5.0 2.0 570 2.62 641 75.00 1.86×10−3 676 0.919 2.00 1.5 6.5 2.7 1 112 3.40 927 75.00 1.68×10−3 1 619 0.981 2.00 3.0 6.5 2.7 393 1.70 654 75.00 2.81×10−3 1 396 0.963 2.00 6.0 6.5 2.7 144 0.85 478 75.00 4.97×10−3 1 201 0.949 5.00 1.5 6.5 2.7 377 3.40 943 90.00 1.70×10−3 1 479 0.967 10.00 1.5 6.5 2.7 145 3.40 765 95.00 0.76×10−3 1 683 0.940 2.02(养殖尾水) 1.5 6.5 2.7 589 3.40 497 75.25 1.82×10−3 1 051 0.955 -
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