Becskereki Gergely, Horvai George, Tóth Blanka
Department of Inorganic and Analytical Chemistry, Faculty of Chemical Engineering and Biotechnology, Budapest University of Technology and Economics, Műegyetem rkp. 3., H-1111 Budapest, Hungary.
Polymers (Basel). 2022 Aug 3;14(15):3167. doi: 10.3390/polym14153167.
Environmental monitoring and remediation often requires the collection of harmful substances from aqueous solutions. Absorption with solids is a useful technique for binding such substances even at very low concentration levels. Many of these contaminants are weak acids or bases. Some novel, nonionic polymeric sorbents, such as hypercrosslinked polymers or polymers with balanced hydrophilic-lipophilic properties (HLB) have been found to bind weak acids and bases with high distribution coefficients even at pH values where these compounds are almost completely ionized (typically near pH 7). To understand this phenomenon and its practical consequences, we have experimentally studied the adsorption of ionizable weak acids and bases as a function of pH and ionic strength on a the OASIS HLB sorbent. Not surprisingly, the ionic forms of the weak acids and bases were found to be much less bound in the aqueous solution than their neutral forms. In spite of this, OASIS HLB binds weak acids and bases around pH 7 considerably better than typical hydrophobic sorbents. The high overall distribution coefficients around pH 7 could be explained by two factors. One is that on OASIS HLB, and on some other novel polymeric sorbents, the binding constant of the moderately hydrophobic neutral form is on the order of 100,000, i.e., much higher than on typical hydrophobic sorbents. Thus, even if the proportion of the neutral form in solution is only around 1% near pH 7, the adsorption of the neutral form is still significant. On the other hand, the binding of the apparently hydrophilic ionized forms occurs with distribution coefficients well above 100. The distribution coefficient of the ionic form appears to depend on ionic strength and the presence of competing ions. Adsorption of the ionic forms is found to be very similar to the adsorption of ionic surfactants. The pH dependence of the total adsorption of neutral and ionic forms together, is found to be steep around pH 7, and therefore the varying pH of natural waters may strongly influence the binding efficiency in practical applications, such as the collection (concentration) of contaminants or their passive sampling.
环境监测与修复通常需要从水溶液中收集有害物质。固体吸附是一种即使在极低浓度水平下也能结合此类物质的有用技术。这些污染物中有许多是弱酸或弱碱。已发现一些新型非离子聚合物吸附剂，如超交联聚合物或具有平衡亲水 - 亲脂特性（HLB）的聚合物，即使在这些化合物几乎完全电离的pH值（通常接近pH 7）下，也能以高分配系数结合弱酸和弱碱。为了理解这一现象及其实际影响，我们通过实验研究了可电离弱酸和弱碱在OASIS HLB吸附剂上的吸附情况，该吸附情况是pH值和离子强度的函数。不出所料，发现弱酸和弱碱的离子形式在水溶液中的结合程度远低于它们的中性形式。尽管如此，OASIS HLB在pH 7左右对弱酸和弱碱的结合能力明显优于典型的疏水吸附剂。pH 7左右较高的总分配系数可由两个因素解释。一是在OASIS HLB以及其他一些新型聚合物吸附剂上，中等疏水性中性形式的结合常数约为100,000，即远高于典型疏水吸附剂上的结合常数。因此，即使在pH 7附近溶液中中性形式的比例仅约为1%，中性形式的吸附仍然很显著。另一方面，明显亲水的离子形式的结合分配系数远高于100。离子形式的分配系数似乎取决于离子强度和竞争离子的存在。发现离子形式的吸附与离子表面活性剂的吸附非常相似。发现中性和离子形式的总吸附对pH的依赖性在pH 7左右很陡峭，因此天然水体变化的pH值在实际应用中可能会强烈影响结合效率，如污染物的收集（浓缩）或其被动采样。
