纳米矿物材料净化甲醛污染的研究进展

Research Progress on Application of Nanominerals in Formaldehyde Removal

甲醛(HCHO)是主要的室内空气污染物之一,对眼睛、鼻腔、呼吸道粘膜组织具有很强的刺激性和毒性。目前,吸附法和非均相催化法是应用最广泛的甲醛净化技术。合成吸附剂如活性炭及其各种改性产品存在甲醛吸附选择性差、合成方法复杂且成本较高等问题。贵金属或过渡金属等合成催化剂虽然对甲醛具有降解效率高、不产生毒副产物等优势,但贵金属价格昂贵,过渡金属催化剂也常需热源才能完全降解甲醛,这限制了其广泛应用。随着对纳米矿物材料环境属性的认识不断加深,纳米矿物材料凭借其天然多孔结构、化学稳定性高、表面酸碱性质可调、资源丰富、价格低廉、处理简单等特性,在甲醛净化中具有潜在的优势。然而,不同种类的纳米矿物材料结构与性质也存在差异,因此其对甲醛的吸附或催化反应活性与作用机制也各不相同。天然硅酸盐类纳米矿物材料如海泡石、凹凸棒石、蒙脱石等由于表面具有酸性位点和较高的表面积,对强极性甲醛分子具有一定的吸附能力。然而,未经处理的天然环境中矿物材料存在吸附选择性差、杂质含量多、孔隙少、表面活性官能团数量少等问题,需要对其进行改性处理。目前,天然纳米矿物吸附剂的改性方法主要有酸处理、焙烧活化和引入有机官能团等。酸、热改性可改善纳米矿物如凹凸棒石、硅藻土的表面酸性,增加其孔隙和孔道开放度,从而增大比表面积。在矿物表面引入氨基等有机官能团可将对甲醛的物理吸附转化为化学吸附,并将甲醛矿化为亚胺类物质。合成纳米矿物吸附剂如沸石分子筛具有孔道结构可调、表面积大等特点,也被广泛应用于甲醛的吸附处理,但是甲醛与水汽存在竞争性吸附,且其抗水性能差,这限制了该类材料的进一步应用。相比于吸附法处理甲醛,非均相催化法如光催化、催化氧化可直接将甲醛转化为二氧化碳和水,具有更加广阔的应用前景。羟基磷灰石、水钠锰矿等纳米矿物对甲醛具有一定催化氧化作用,通过掺杂金属元素、调控其结构与形貌可进一步提高其对甲醛的催化氧化能力。纳米黏土矿物如凹凸棒石、海泡石可作为载体与贵金属、过渡金属、纳米二氧化钛等活性组分复合,从而改善活性组分的分散性,提供反应羟基等活性位点,显著提升复合矿物材料的催化性能,在低温乃至室温下就可将甲醛完全降解。本文综述了各种纳米矿物材料在吸附和非均相催化降解室内空气和部分水体中甲醛的研究进展,系统比较了纳米矿物材料对甲醛的吸附、催化性能、反应机理,指出了该材料目前研究存在的问题,并对其今后的发展趋势进行了展望。

Formaldehyde is the major indoor pollutant due to its respiratory tract irritation and human toxicity.The conventional formaldehyde removal technology includes adsorption and heterogeneous catalytic oxidation.Synthetic adsorbents such as activated carbon and its various modified products have some defects of poor formaldehyde adsorption selectivity,complicated synthesis methods,and high cost.Synthetic catalysts such as precious metals or transition metals have the advantages of high catalyticactivity of formaldehyde and no toxic by-products,but,the precious metals are expensive,and the transition metal catalysts commonly require a heat source,which limits their wide application.With the deepening of understanding on the environmental properties of nanominerals in recently years,the nanominerals with the advantages of natural porous structure,chemical stability,adjustable surface acidity,abundant resources,low price,and easy handling have potential applications on removal of HCHO.However,the nanomineral materials with different structures and properties have a various function and reaction mechanism for HCHO removal.The acid sites and high specific surface area enhanced the adsorption performance of natural silicate nanominerals(e.g.,sepiolite,palygorskite,and montmorillonite).However,poor adsorption selectivity,high impurity content,low porosity and surface-active functional groups of silicate nanominerals restricted their further development.The primary modification methods of nanomineral mainly include acid,thermal activation,and the introduction of organic functional groups.Acid and thermal activation can improve the acidity,pore structure,and the specific surface area of nanominerals,such as palygorskite and diatomite.The absorbed formaldehyde can be mineralized on the absorbent surface by the introduction of amino groups.Zeolite molecular sieves are also widely used in formaldehyde adsorption due to their controllable pore structure and large surface area.However,water vapor adsorbs competitively with HCHO molecules on active sites and suppress the performance for formaldehyde removal,and hence limited the applicability of zeolite molecular.Compared with the adsorption method for processing formaldehyde,heterogeneous catalytic methods such as photocatalysis and catalytic oxidation can directly convert formaldehyde into carbon dioxide and water,and have a broader application prospect.Nanominerals such as hydroxyapatite and bimanganite have a certain catalytic oxidation effect on formaldehyde.By doping metal elements and adjusting their structure and morphology,their catalytic oxidation capacity for formaldehyde can be further improved.Nanominerals such as palygorskite and sepiolite as carriers compounded with precious metals,transition metals,or nano-titanium dioxide can improve the distribution of active components,provide more active sites such as reactive hydroxyl groups and significantly improve the catalytic performance of composite mineral materials.The above-mentioned composite minerals catalysts even can catalytic oxidation of formaldehyde at ambient temperature.The research progress of adsorption and heterogeneous catalytic oxidation of formaldehyde by nanominerals is reviewed,the formaldehyde removal performance,reaction mechanism,and current problems are investigated.Finally,the application of nanominerals for formaldehyde remo-val based on achievements is proposed.