Heavy Metal Release from Soils in Batch pHstat Experiments

BC of soils (van Breemen et al., 1983). Soil acidification may be defined as the loss of ANC and does not necesIn Slovakia H input into soils is increasing because alkaline dust sarily result in the decrease of the soil equilibrium pH emissions have been reduced since 1990. Our objectives were to exam(van Breemen et al., 1983). Süsser (1987) described ine (i) the metal release in H buffer reactions of Slovak soils and (ii) the impact of the proton buffering on heavy metal partitioning. buffer reactions in soils during pHstat experiments as the We used 10 soils (pH 7.4–3.6) in a batch pHstat experiment. Released sum of two independent reactions both following firstions were adsorbed to an ion-exchange resin, while pH was kept order kinetics. constant. After reaction times of 10 min, 30 min, 1, 2, 4, 24, 48, Batch and column techniques are used to examine and 96 h, soil and ion-exchange resin were separated and ions were BCs and reaction kinetics (Selim and Amacher, 1997). extracted from the resin. After 0, 4, and 96 h, we determined metal In automatic titroprocessors, the proton activity in the concentrations in seven fractions. On average, the total release after reaction solution is kept constant at a higher level than 96 h (in percentage of the total concentration) decreased in the order: soil equilibrium pH by measuring pH continuously and Cd (74) . Pb (59) . Cu (29) . Zn (22) . Ni (17) . Cr (3.1), while replacing buffered protons by the addition of mineral the initial release rate into solution decreased in the order: Cd . acid (Süsser, 1987; van de Sand and Fischer, 1994). Ions Zn . Ni . Cu . Pb . Cr. After 48 h, only the Cr release rate was lower than that of Cd. Particularly in less acid soils, Pb, Cu, Zn, and released into solution during buffering are not removed Cd released by the dissolution of oxides were adsorbed onto the soil from the system and therefore affect further reactions matrix, explaining the slower decrease of the Pb and Cu release (Kaupenjohann and Wilcke, 1995a). When a convenrates. The percentages of weakly bound heavy metals decreased more tional titroprocessor is used, for example, the cumulamarkedly during the titration than those of metals bound to Fe oxides. tive Fe release reaches a constant value after a short Increased H inputs into Slovak soils will cause enhanced metal release period of time, which is probably controlled by the soluinto soil solution. bility product of Fe oxides (Kaupenjohann and Wilcke, 1995b). When an ion exchanger is used, the products of the buffer reactions are removed from the solution M soils in industrialized countries are afand the pH is kept constant due to equivalent proton fected by acid depositions from the atmosphere release (Kaupenjohann and Wilcke, 1995b). (Reuss and Johnson, 1986). In Slovakia and other cenSoil acidification can damage fine roots and soil ortral and eastern European countries, acid depositions ganisms (Rehfuess, 1981). The leaching of nutrients rewere often neutralized by the simultaneous emissions leased during buffer reactions may cause nutrient defiof alkaline fly ashes (Draaijers et al., 1995; Semb et al., ciency in plants (Kaupenjohann, 1989). Aluminum and 1995). As a consequence of the political changes in the heavy metal concentrations may reach concentrations beginning of the 1990s, alkaline dust emissions have toxic to plants or may result in the enhanced metal been reduced, resulting in an increasing acidity of the uptake by plants, thus accumulating in food chains depositions (Hedin et al., 1994; Semb et al., 1995). (Tyler et al., 1989), but only a few studies have focused The introduced protons result in many buffer reacon heavy metal release in proton buffer reactions tions in the soils (Matzner and Ulrich, 1984; Schwert(Kaupenjohann and Wilcke, 1995a). mann et al., 1987). While the reduction of variable The objectives of this study were (i) to examine the charge at the surface of clay minerals and humic submetal release in H buffer reactions of Slovak soils stances is reversible, carbonates, silicates, oxides, and varying in pH and (ii) to assess the impact of proton hydroxides are dissolved irreversibly (Ulrich, 1981; Süsbuffering on heavy metal partitioning in these soils. ser, 1987). Heavy metals associated with the dissolved compounds, are released into the soil solution (KaupenMATERIALS AND METHODS johann and Wilcke, 1995a). Study Sites and Soils The acid-neutralizing capacity (ANC), which equals the sum of the buffer capacities (BC) of all individual We sampled ten A horizons in central Slovakia. The landbuffer systems, is an often-used measure of the proton use type of the sites are shown in Table 1. The soil pH values of Samples 1, 2, 3, and 4 are governed by the buffering of silicates (neutral to weakly acid soils), those of Samples 5, 6, Andreas Schwarz, Wolfgang Wilcke, and Wolfgang Zech, Institute and 7 by the buffering of exchangeable cations (acid soils), of Soil Science and Soil Geography, Univ. of Bayreuth, D-95440 and those of Samples 8, 9, and 10 by the buffering of Al oxides Bayreuth, Germany. Jan Stýk, Soil Fertility Research Institute, Mlade(very acid soils; Ulrich, 1981). znicka 36, SK-97400 Banska Bystrica, Slovakia. *Corresponding author ([email protected]). Abbreviations: ANC, acid-neutralizing capacity; BC, buffer capacity; ECEC, effective cation-exchange capacity; PE, polyethylene. Published in Soil Sci. Soc. Am. J. 63:290–296 (1999).