重金属盐溶解-沉淀反应的吉布斯自由能变的定量估算

Quantitative calculation of the change of Gibbs' free energy in dissolution and precipitation reactions of heavy metal ions

对重金属盐溶解-沉淀反应的吉布斯自由能变(ΔG)进行了定量估算;在不加缓冲剂和pH调节剂的纯水体系中,观测了重金属离子Pb2+、Ni 2+及Zn2+的沉淀-溶解反应,计算了反应的表观平衡常数Kf及表观溶度积常数K′sp;与此同时,对相关计算结果进行了实验验证.结果表明,重金属盐溶解-沉淀反应的ΔG只与反应物的初始浓度和平衡浓度有关,通过测定反应物的平衡浓度即可求得反应的ΔG;重金属离子Pb2+、Ni 2+及Zn2+的沉淀-溶解反应表观平衡常数Kf及表观溶度积常数K′sp皆随重金属盐的起始表观浓度Q0的增大而发生规律性变化:Kf先增大后减小,K′sp趋于最大恒定值,并接近于Ksp;而三种体系的ΔG均为负值,ΔG的绝对值先随Q0的增大而增大,然后趋于相对恒定.实验结果表明,不能用Kf而只能用Ksp描述重金属盐的沉淀-溶解平衡,ΔG绝对值与真正参与反应的反应物的量呈正相关;ΔG<0说明纯水中重金属盐的溶解反应是自发进行的.类似地,通过计算溶解-沉淀反应的ΔG,可以判断沉淀法处理重金属污染废水在热力学上的可行性.

The change in the Gibbs＇ free energy （AG） of the dissolution-precipitation reaction of heavy metal salts was quantitatively estimated. The dissolution-precipitation reactions of heavy metal ions Pb2＋ , Ni2＋ and Zn2＋ in pure water without buffer and pH-adjusting agent were in- vestigated, while the apparent equilibrium constant （Ks） and the apparent solubility product constant （Krsp） of the dissolution-precipitation reactions were calculated. In the meantime, relevant calculation results were verified by experiments. Results indicate that the AG of the dissolution-precipitation reaction of heavy metal salts is only dependent on the initial concentra- tion and equilibrium concentration of reactants, and it can be calculated by determining the equilibrium concentration of the reactants. The Kf and K＇5p of the precipitation-dissolution re- actions of heavy metal ions Pb2＋ , Ni2＋ and Zn2＋ in aqueous system vary with increasing appar- ent initial concentration （Q0） of the heavy metals in a regular manner. Namely, Ks tends to rise initially but then stabilize with increasing Q0, and K＇5p tends to reach the maximum constant and become close to Ksp therewith. Besides, the calculated AG of the three systems is negative, and the absolute value of AG rises with increasing Q0 and tends to reach a constant. Relevant experimental results indicate that the precipitation and dissolution reactions of heavy metal saltscannot be described by Kf but by K5p, and the absolute value of AG is positively proportional to the quantity of the reactant really participating in reaction. Moreover, the spontaneity of the precipitation-dissolution reaction of heavy metal salts can be discussed using calculated AG. Similarly, the feasibility to use precipitation method to treat wastewater containing heavy metal ions can be judged by calculating the AG of the dissolution-precipitation reaction.