College of Environmental and Resource Science, Zhejiang University, Hangzhou, China.
School of Environmental Science and Engineering, Shandong University, Qingdao, China.
Environ Sci Process Impacts. 2021 Feb 4;23(1):170-178. doi: 10.1039/d0em00430h.
Hydroxypyromorphite (HPM) is a low-solubility Pb phosphate mineral that has the potential to limit solubility and bioavailability of Pb in soils and water. Because of reported uncertainty regarding the solubility product of this important mineral, we re-evaluated the solubility of Pb and activity of the free Pb2+ ion in aqueous suspensions of microcrystalline HPM equilibrated up to 30 days over a wide range of added soluble phosphate. A small addition of phosphate (0.1 mM) reduced Pb solubility as measured by ICP-OES, but greater phosphate additions (up to 50 mM) had no further effect in lowering HPM solubility. However, free Pb2+ ion activity measured by ion-selective electrode progressively decreased from about 10-6.5 with no added phosphate to 10-9 as soluble phosphate was increased. The effect of soluble phosphate in lowering Pb2+ activity is attributed to inhibited dissolution of HPM as well as increased Pb2+-phosphate ion pair formation in solution at higher solution concentrations of phosphate. Measurement of the ion activity products (IAP) of the solutions at equilibrium with HPM gave highly variable IAP values that were sensitive to pH and were generally not consistent with the reported solubility product of this mineral. The high variability of the IAPs for solutions with variable pH and phosphate concentrations indicates that dissolution-precipitation reactions of HPM are not described by a constant solubility product at equilibrium, possibly because of the incongruent dissolution behavior of this mineral at near-neutral pH.
羟磷铅矿（HPM）是一种低溶解度的 Pb 磷酸盐矿物，它有可能限制土壤和水中 Pb 的溶解度和生物可利用性。由于报道称这种重要矿物的溶度积存在不确定性，我们重新评估了在添加可溶性磷酸盐的范围内，在长达 30 天的时间内，微结晶 HPM 水悬浮液中 Pb 的溶解度和游离 Pb2+离子的活度。少量磷酸盐（0.1mM）的添加会降低 ICP-OES 测量的 Pb 溶解度，但更大的磷酸盐添加量（高达 50mM）在降低 HPM 溶解度方面没有进一步的影响。然而，通过离子选择性电极测量的游离 Pb2+离子活度从没有添加磷酸盐时的约 10-6.5 逐渐降低到添加可溶性磷酸盐时的 10-9。可溶性磷酸盐降低 Pb2+活度的作用归因于 HPM 的溶解抑制以及在较高磷酸盐溶液浓度下，溶液中 Pb2+-磷酸盐离子对的形成增加。与 HPM 平衡的溶液的离子活度积（IAP）的测量给出了高度可变的 IAP 值，这些值对 pH 敏感，并且通常与该矿物的报道溶解度积不一致。pH 和磷酸盐浓度变化的溶液的 IAPs 的高变异性表明，HPM 的溶解-沉淀反应不能用平衡时的恒定溶解度积来描述，可能是因为这种矿物在近中性 pH 下的不相溶溶解行为。
