Search

Abstract: Radio pollutant removal is one of several priority restoration strategies for the environment. This study assessed the effect of low molecular weight organic acid on the lability and mechanisms for release of 137Cs from contaminated soils. The amount of 137Cs radioactivity released from contaminated soils reacting with 0.02M low molecular weight organic acids (LMWOAs) specifically acetic, succinic, oxalic, tartaric, and citric acid over 48h were 265, 370, 760, 850, and 1002Bqkg−1, respectively. The kinetic results indicate that 137Cs exhibits a two-step parabolic diffusion equation and a good linear relationship, indicating that the parabolic diffusion equation describes the data quite well, as shown by low p and high r 2 values. The fast stage, which was found to occur within a short period of time (0.083–3h), corresponds to the interaction of LMWOAs with the surface of clay minerals; meanwhile, during the slow stage, which occurs over a much longer time period (3–24h), desorption primarily is attributed to inter-particle or intra-particle diffusion. After a fifth renewal of the LMWOAs, the total levels of 137Cs radioactivity released by acetic, succinic, oxalic, tartaric, and citric acid were equivalent to 390, 520, 3949, 2061, and 4422Bqkg−1 soil, respectively. H+ can protonate the hydroxyl groups and oxygen atoms at the broken edges or surfaces of the minerals, thereby weakening Fe–O and Al–O bonds. After protonation of H+, organic ligands can attack the OH and OH2 groups in the minerals easily, to form complexes with surface structure cations, such as Al and Fe. The amounts of 137Cs released from contaminated soil treated with LMWOAs were substantially increased, indicating that the LMWOAs excreted by the roots of plants play a critical role in 137Cs release. [Copyright &y& Elsevier]

Soil respiration represents the second largest carbon flux, next to photosynthesis of the terrestrial biosphere, and thus plays a dual role in regional and global carbon cycles. However, soil respiration in Asian monsoon forests with high rainfall has rarely been studied. In this study, we continuously measured soil respiration using a 12-channel automated chamber system in a 61-year-old Japanese cedar forest in central Taiwan with annual rainfall greater than 2500 mm. A 4-year (2011–2014) continuous half-hourly dataset was used to quantify the influences of soil temperature and moisture, especially rainfall events, on both total soil respiration (Rs) and heterotrophic respiration (Rh). The annual mean Rs was approximately 10.8 t C ha−1 (ranging from 10.7 to 10.9) t C ha−1, with Rh contributing approximately 74.6% (ranging from 71.7% to 80.2%). Large seasonal variations in both Rs and Rh were primarily controlled by soil temperature. Over 45.8% of total annual rainfall amounts were provided by strong rainfall events (over 50 mm), and over 40% of rainfall events occurred during summers between 2012 and 2014. These strong rainfall events caused rainwater to enter soil pores and cover the soil surface, which resulted in limited soil microorganism activity and, consequently, restricted CO2 production. The mean Q10 values were 2.38 (ranging from 1.77 to 2.65) and 2.02 (ranging from 1.71 to 2.34) for Rs and Rh, respectively. The Q10 values in this study, which were lower than in global forest ecosystems, may imply that the interannual Rs values observed in this study that were caused by high rainfall were less temperature-dependent than the Rs levels in global forest ecosystems. Both Rs and Rh were negatively correlated with soil moisture, which indicated that the soil moisture levels in the studied forest were usually under saturated conditions. These results also provide the lack of data for respiration in the Asian monsoon region under high-rainfall conditions. [ABSTRACT FROM AUTHOR]

Abstract: Arsenate is one of the main contaminants threatening water supplies all over the world. Layered double hydroxides (LDHs) have large surface areas and high anion exchange capacities, so they may be used to develop rapid and cost-effective methods for scavenging As from contaminated water. In order to enhance arsenate adsorption of layered double hydroxides, it is essential to have a fundamental understanding of the interactions between arsenate and LDHs. In this study, the effects of layer charge density and interlayer nitrate orientation of Mg/Al–NO3 LDHs on the preferential adsorption of arsenate were investigated. Mg/Al–NO3 LDHs with a Mg:Al ratio of 2:1, 3:1 and 4:1 were synthesized, using a constant-pH co-precipitation method. The adsorption maxima of As on 2:1, 3:1 and 4:1 LDHs were 1.56, 1.08 and 0.36 mmol g−1, respectively, following the order of their layer charge densities. In addition, nitrate orientation in the LDHs plays a significant role in determining arsenate adsorption. The low arsenate adsorption of 4:1 LDH relative to its anion exchange capacity resulted from the low degree of access arsenate has to the interlayer space, so that arsenate adsorption largely occurred on the external surface of the material. The accessibility of the interlayer of 4:1 LDH to arsenate was restricted by the small basal spacing as a result of the parallel orientation of the interlayer nitrate with respect to the hydroxide sheets. Conversely, interlayer nitrate with an orientation perpendicular to the hydroxide sheets in 2:1 and 3:1 LDHs can be exchanged for arsenate more readily. Consequently, these two LDHs exhibit greater arsenate adsorption capacity, because arsenate can be adsorbed on both their external and interlayer surfaces. The arsenate adsorption of the LDHs also was highly dependent upon the type and concentration of competing anions. The negative effect of the competing anions on arsenate adsorption by various LDHs generally followed the order H2PO4 − >HCO3 − >SO4 2− >F− >Cl−. Due to the difference in adsorption mechanism, the arsenate adsorption of 4:1 LDH is subject to the negative impact of competing anions to a greater extent than those of 3:1 and 2:1 LDH. These results demonstrated that the preferential adsorption of arsenate by LDHs is dependent upon the nitrate orientation in these materials, as a consequence of changing layer charge density. The selective retention of arsenate for wastewater treatment can be maximized by controlling the orientation of interlayer nitrate in Mg/Al–NO3 LDHs. [Copyright &y& Elsevier]

The objective of this study was to provide fundamental knowledge of arsenate sorption on lithium/aluminum layered double hydroxide intercalated by chloride (Li/Al LDH-Cl) and further to reveal the contribution of exposed positive charge surface of Li/Al LDH-CI created by intercalating LiCl into Al(OH)3 layers to arsenate sorption. Therefore, sorption isotherms, envelopes and extended X-ray absorption fine structure (EXAFS) technique were employed to examine the reaction of arsenate on Li/Al LDH-Cl and on gibbsite. Based on an isotherm study, the sorption maximum of Li/Al LDH-Cl for arsenate was approximately six times higher than that of gibbsite. Sorption envelopes of arsenate on Li/Al LDH-Cl displayed a pH-sensitive behavior from pH 4.0 to 7.0, but it was insensitive to pH above pH 7.0, approaching to the pHpzc of Li/Al LDH-Cl (7.22). This transformation with shifted pHs illustrated that there were two types of reaction sites within Li/Al LDH-Cl that participate in arsenate sorption; one is pH-sensitive and the other is not. From EXAFS analysis, arsenate sorbed on Li/Al LDH, reacted not only with Al in the edges of Al-(OH)3 layers, but also with Li located in the vacant octahedral sites within Al(OH)3 layers; however, the decreasing intensity of As(V)-Al shells with increasing pH represented there were fewer As(V)-Al complex existed at higher pH, i.e., the complex between arsenate and Al is pH-sensitive. The superior sorption capability of Li/Al LDH-Cl to that of gibbsite could be attributed to the intercalated Li cations which served as the permanent sorption sites and made the surface of Al(OH)3 have high affinity to arsenate.