Mechanochemistry: A review of surface reactions and environmental applications

Mechanochemistry is the study of physico-chemical transformations generated by mechanical force. This force may break down crystals, thus exposing fresh, active surfaces and enhance the mass transfer required for reaction partners in the solid state to make the contact required for initiating a chemical reaction. Application of mechanical force may facilitate sorption-induced steric enhancement of transformations catalyzed by ions of transition metals (e.g., cations of such metals serving as counter ions in exchange complexes of clays). The formation of mixed Cu–Na–montmorillonite from Na–montmorillonite ground with CuCl2 in the presence of imazaquin and the resultant breakdown of imazaquin is one example of this phenomenon. Reported mechanochemical reactions which involve minerals, include redox reactions, polymerization and polymer rearrangements, recrystallization and dehydration. The more widespread methods for removing organic contaminants from soils are based on biological degradation. Biological processes, however, have certain shortcomings such as the substantial time they require, or the significant concentrations of harmful residues that are left in the soil. Alternative soil remediation schemes are, therefore, needed and mechanochemical procedures may be part of such innovative cleanup protocols. Mechanochemistry can be applied at the field scale and mechanochemical techniques were already suggested as a clean and cheap approach to hazardous waste management and destruction. Furthermore, soil tillage is actually a process in which mechanical force is applied to the soil and hence, it may induce a variety of mechanochemical reactions. Clays and metal (especially iron and manganese) oxides catalyze numerous transformations of organic compounds when such compounds are sorbed on the minerals' surfaces. The catalytic efficiency of these minerals is, at least in part, due to their strong acidity, both Bronsted and Lewis, and to the large specific surface area they may possess. Manganese and iron are redox active and their minerals are thus capable of supporting both electron and proton transfer reactions. Since iron oxides are abundant in soils, transformations augmented by these minerals are of considerable environmental importance. In the present review, case studies of mechanochemically induced transformations of pollutants are presented. The breakdown of pesticides (2,4-D, imazaquin and atrazine) and of other organic pollutants such as DCP and PAHs achieved by light grinding for short durations (a few minutes) in a mixture with minerals which are soil components or the derivatives of such components is described. Among the tested minerals, Al– and Cu–montmorillonite, iron oxides and Mn oxides were the most effective heterogeneous catalysts. The effectiveness of the minerals depended strongly on the degraded pollutant and on the conditions (e.g., the moisture content) under which the mechanochemical reaction took place. Although the presence of water often hinders the degradation, in some cases the reverse is true. Na–montmorillonite in the presence of CuCl2 was effective in degrading imazaquin when the mixture was ground wet. This demonstrates a detoxification process applicable to real-world systems which are often wet.