Impacts of wet-dry alternations on cadmium and zinc immobilisation in soil remediated with iron oxides

Chemical immobilisation is extensively used for in-situ remediation of heavy metals contaminated soil. Immobilised heavy metals could be reactivated by multiple factors such as pH, moisture, temperature, rainfall, etc., among which rainfall is very important, especially acid rain in southern China. Wet-dry alternations were used to simulate the leaching of metals by rainwater. The variation of cadmium (Cd) and zinc (Zn) speciation distribution in soil immobilised with iron oxides (goethite (GE) and 2-line ferrihydrite (GLS)) was investigated. The impacts of wet-dry alternations on the properties of the soil and amendments were also assessed. In the soil without amendments (OS) and amended with GE (GS), the stable fractions were reactivated and transformed into labile fractions under wet-dry alternations. In the soil amended with GLS (LS), the exchangeable and carbonate-bound Cd decreased while the soluble, Fe-Mn oxide bound and organic bound Cd increased. The carbonate-bound Zn was transformed into the Fe-Mn oxide-bound Zn. Transformation from the amorphous iron oxide into crystalline iron oxide in GS and LS were 4.9% and 5.3%. The Pearson correlation analysis showed that the soil pH and the iron-oxide speciation were strongly correlated with Cd/Zn fractions in the soil. The specific surface area, pore volume and adsorption capacity of the iron oxides decreased by 9.26%, 38.89% and 62-73% (for GE), 1.88%, 22.22% and 26-55% (for GLS). The altered soil properties and morphological differences between the two iron oxides under wet-dry alternations were important reasons for Cd/Zn reactivation.

The effects of H2O2- and HNO3/H2SO4-modified biochars on the resistance of acid paddy soil to acidification

Biochar was prepared from rice straw and modified with 15% H₂O₂ and 1:1 HNO₃/H₂SO₄, respectively. The unmodified biochars and HCl treated biochars for carbonate removal were used as control. The biochars were added to the acid paddy soil collected from Langxi, Anhui Province, China at the rate of 30 g/kg. The paddy soil was flooded and then air-dried, and soil pH and Eh were measured in situ with pH electrode and platinum electrode during wet-dry alternation. Soil pH buffering capacity (pHBC) was determined by acid-base titration after the wet-dry treatment. Then, the simulated acidification experiments were carried out to study the changing trends of soil pH, base cations and exchangeable acidity. The results showed that soil pHBC was effectively increased and the resistance of the paddy soil to acidification was apparently enhanced with the incorporation of H₂O₂- and HNO₃/H₂SO₄-modified biochars. Surface functional groups on biochars were mainly responsible for enhanced soil resistance to acidification. During soil acidification, the protonation of organic anions generated by dissociation of these functional groups effectively retarded the decline of soil pH. The modification of HNO₃/H₂SO₄ led to greater increase in carboxyl functional groups on the biochars than H₂O₂ modification and thus HNO₃/H₂SO₄-modified biochars showed more enhancement in soil resistance to acidification than H₂O₂-modified biochars. After a wet-dry cycle, the pH of the paddy soil incorporated with HNO₃/H₂SO₄-modified biochar increased apparently. Consequently, the addition of HNO₃/H₂SO₄-modified biochar can be regarded as a new method to alleviate soil acidification. In short, the meaning of this paper is to provide a new method for the amelioration of acid paddy soils.

Influences of the alternation of wet-dry periods on the variability of chromophoric dissolved organic matter in the water level fluctuation zone of the Three Gorges Reservoir area, China

Dissolved organic matter (DOM) is a crucial driver of various biogeochemical processes in aquatic systems. Thus, many lakes and streams have been investigated in the past several decades. However, fewer studies have sought to understand the changes in DOM characteristics in the waters of the Three Gorges Reservoir (TGR) areas, which are the largest artificial reservoir areas in the world. Thus, a field investigation of dissolved organic carbon (DOC) concentrations and of chromophoric dissolved organic matter (CDOM) properties was conducted from 2013 to 2015 to track the spatial-temporal variability of DOM properties in the TGR areas. The results showed that the alternations of wet and dry periods due to hydrological management have a substantial effect on the quantity and quality of aquatic DOM in TGR areas. Increases in DOC concentrations in the wet period show an apparent "dilution effect" that decreases CDOM compounds with relatively lower aromaticity (i.e., SUVA₂₅₄) and molecular weight (i.e., S_R). In contrast to the obvious temporal variations of DOM, significant spatial variability was not observed in this study. Additionally, DOM showed more terrigenous characteristics in the dry period but weak terrigenous characteristics in the wet period. Furthermore, the positive correlation between SUVA₂₅₄ and CDOM suggests that the aromatic component controls the CDOM dynamics in TGR areas. The first attempt to investigate the DOM dynamics in TGR areas since the Three Gorges Dam was conducted in 2012, and the unique patterns of spatial-temporal variations in DOM that are highlighted in this study might provide a new insight for understanding the role of DOM in the fates of contaminants and may help in the further management of flow loads and water quality in the TGR area.