Tính axit-không đổi hành vi hiệu quả Gần Điều kiện Zero-Charge Mô hình hiện tại địa hóa cho mô hình hóa các hành vi phân vùng rắn / nước của dấu vết loài độc hại ion ở nồng độ hòa tan khoáng sản porewater subsaturation dựa vào hai cơ chế cơ bản: (1) hình thành giải pháp vững chắc với các giai đoạn của các phần tử rắn hiện tại chủ yếu trong môi trường, và (2) hấp phụ phản ứng trên bề mặt môi trường. Hình thành dung dịch rắn là quá trình dẫn đến sự thay thế của một. | 11 Effective Acidity-Constant Behavior Near Zero-Charge Conditions NICHOLAS T. LOUX . Environmental Protection Agency Athens Georgia . I. INTRODUCTION Current geochemical paradigms for modeling the solid water partitioning behavior of trace toxic ionic species at subsaturation mineral solubility porewater concentrations rely on two fundamental mechanisms 1 solid solution formation with the major element solid phases present in the environment and 2 adsorption reactions on environmental surfaces. Solid solution formation is the process leading to the substitution of a trace ion for a major ion in a natural solid phase . Ref. 1 . For example solid solution formation between Cr3 and Fe OH 3 has been reported in the literature as a possible porewater solubility-limiting mechanism for dissolved Cr3 . This reaction can be described by nCr3 Fe OH 3 nFe3 Fe i- Cr OH 3 where n 1 2 . The second mechanism the topic of this chapter is generally believed to be more widespread in environmental systems and is frequently described as the result of surface complexation reactions between ionizable species Mez and reactive surface sites SOH present on environmental solids including iron oxides manganese oxides aluminum oxides silicon oxides aluminosilicates and particulate organic carbon. For example a reaction of the form Mez SOH SOMe z 1 H can be described by the following generic mass action expression . see Ref. 3 and applications in Ref. 4 Krxn SOMe z 1 a H UAG excess Rr 1 Me z SOH Copyright n 2003 by Marcel Dekker Inc. All Rights Reserved. where Krxn formation constant for the rxn a H bulk solution H chemical activity z valence of cation R gas constant aMe z bulk solution metal ion activity SOMe z 1 concentration of complexed sites e base of natural logarithm AG excess excess free energy T absolute temperature SOH concentration of unbound sites Equation 1 differs from a solution counterpart in two ways 1 Analogous to surface protonation reactions Eq. 1 is a mixed