铁氧化物矿物对苯酚和溶解性有机质表面吸附的初步研究

文中以铁氧化物矿物对苯酚和溶解性有机质(DOM)的吸附研究为例开展生态矿物学研究。铁氧化物矿物的吸附作用存在多种机制,这些吸附机制发生作用的条件主要取决于溶液化学性质和吸附质的理化性质。批处理实验研究表明,苯酚吸附在酸性微酸性条件下不强,吸附等温线符合Langmuir方程,属于表面分子吸附模型;DOM的吸附强并发生吸附分异,配体交换、憎水键和范氏力等多种模式并存,在酸性中性条件下对DOM在针铁矿上的吸附起着重要贡献。本文实验条件下DOM吸附等温线近于线性,不能采用Langmuir方程拟合,可能原因是DOM浓度较低。矿物表面荷电性对吸附影响显著,例如,当矿物表面净电荷为零(pH=pHpzc)时,矿物表面水化膜减薄甚至消失,苯酚分子、憎水DOM分子或片断都会倾向于在矿物表面上吸附。由于苯酚吸附机制单一,其受到的影响很明显,所以苯酚在pH值7～8范围内出现吸附最大值;由于配体交换作用主要发生在酸性微酸性条件下,所以在本文pH值约7.5的实验条件下,尽管配体交换仍在发生作用,但它不是主要吸附机制,针铁矿对DOM吸附的主要贡献应是憎水键和范氏力作用,此外,DOM吸附等温方程近于线性还可能与此有关。显然,铁矿物表面作用在对有机质含量低而铁矿物含量高的红壤中污染物和DOM的固定与归宿控制中扮演着重要的角色。

A preliminary study of surface adsorption of iron oxide minerals for phenol and DOM

The aim of this article is to research the eco-mineralogical development by investigating the adsorption of iron oxide minerals for phenol and dissolved organic matter(DOM).The multiple mechanisms of surface adsorption associated with the iron minerals can coexist,mainly depending on the solution chemistry as well as the natural and physicochemical properties of adsorbates and minerals.The adsorption study was conducted by batch equilibrium techniques.The results showed that the adsorption of goethite and hematite for phenol is weak in acidic and mild acidic media,and that the phenol adsorption isotherms can be fitted well by Langmuir equation with a correlation coefficient of R>0.985.Therefore,the phenol adsorption was subjected to a surface physical adsorption model,whereas the sorption of goethite for the DOM was stronger along with the DOM sorption fractionation.The DOM sorption fractionation was controlled simultaneously by multi-mechanisms in acidic and neutral media,mainly including "surface ligand exchange","hydrophobic force",and "van der Waals forces" models.Moreover,the DOM sorption isotherms can be fitted well by linear and Freundlich equations with correlation coefficients,R>0.92,but not by Langmuir equation ascribed to low concentration of DOM used in this article.The effect of the mineral surface charge on adsorption was significant,especially when pH of the solution neared pHpzc(pH for point of zero charge);the phenol and the hydrophobic DOM molecule and/or fraction can preferentially adsorb onto iron oxide minerals because of the decrease in thickness;even the binding of the water film over the iron minerals disappear.However,the phenol adsorption can be impacted more significantly because of its simple adsorption model,therefore,its pH-dependence adsorption curve showed a peak value at pH 7-8 near the pHpzc.In general,surface complexation reaction of iron oxides can occur especially in acidic and mild acidic media.However,under the condition of pH 7.5 or so of this study,though the surface ligand exchange may have been still functional,the predominant adsorption mechanisms of DOM onto goethite would be mainly based on the "hydrophobic force" and "van der Waals forces" models.Moreover,the adsorption isotherm of DOM was almost linear(Freundlich equation fitting constant n=1.02),accounting for the importance of the later two types of adsorption models in the DOM adsorption.Of course,the surface sorption reactions of iron oxide minerals play an important role in regulating immobility and fate of pollutants and natural organic matter in red soils that contain low content of organic matter but high content of iron oxide minerals.