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Though Pb(II) adsorption onto HA has been extensively studied, its kinetic and thermodynamic features are not fully understood. This work investigated the kinetic processes and isotherms of Pb(II) adsorption onto a humic acid (HA) derived from leonardite in an aqueous solution. The basic properties of the HA were determined by standard methods, and Fourier transform infrared spectroscopic (FTIR) technique. Adsorption kinetic experiments were conducted at 120 mg Pb/L at 288, 298, 308, and 318 K. The adsorption data were best fitted into the pseudo-second-order model, suggesting the chemisorption nature of Pb(II) adsorption. Batch adsorption experiments were conducted at 0–200 mg Pb/L, and the data fit the Freundlich and Temkin models well. Pb(II) adsorption onto HA initially increased, then decreased, with rising temperature. Thermodynamic parameters showed that Pb(II) adsorption was exothermic and spontaneous. Though Cu(II), Zn(II), or Cd(II) could compete with Pb(II) for adsorption, the low cost and high adsorption capacity of leonardite-derived HA determined that it was an excellent adsorbent to remove Pb(II) from an aqueous solution. The optimized experimental conditions derived from the central composite design (CCD) were 20 mg HA, pH 5.0, 4-h react time, and 80 mg Pb/L.

Heavy metals release from contaminated sediments is one of the most important chemical processes affecting overlying water quality in river, lake, and ocean. The objective of this study was to determine the leaching properties, kinetic rate, and leaching amount of zinc (Zn) and cadmium (Cd) from the river sediment contaminated by gold mining. Speciation of Zn and Cd in the sediments was extracted by a modified BCR extraction procedure. Release kinetics of Zn and Cd were studied by a simulated leaching experiment using a stirred-flow reactor and a two-site equilibrium-kinetic model. The sediments we studied were significantly contaminated by Zn (620–5878 mg kg−1) and Cd (2–67 mg kg−1), and both have high content of weak acid extractable forms. There were much more smaller particles on the surface of sediment JH01 and JH02 than sediment JH03 and JH04. The two-site equilibrium-kinetic model fits the release data of Zn and Cd well, and it was demonstrated as an effective tool to describe the kinetic release of Zn and Cd from river sediments. Kinetic rates obtained from curve fitting showed large variation among sediments indicating different reaction mechanisms. The rapid release stage (before the second stop-flow) of Zn and Cd was controlled both by the equilibrium sites and the kinetic sites, while the slow release stage (after the second stop-flow) was mainly controlled by the kinetic sites. The total leaching amount of Zn and Cd in JH01 (27.1 mg kg−1, 0.4 mg kg−1), JH02 (474 mg kg−1, 7.5 mg kg−1), JH03 (320 mg kg−1, 7.6 mg kg−1), and JH04 (52.4 mg kg−1, 2.0 mg kg−1) demonstrated that large amount of Zn and Cd in sediments can be leached into solution. Thus, effective measures should be taken to prevent leaching of heavy metals from river sediment.

The metal-binding characteristics of water-soluble organic matter (WSOM) emitted from biomass burning (BB, i.e., rice straw (RS) and corn straw (CS)) with Cu(II) under various pH conditions (i.e., 3, 4.5, and 6) were comprehensively investigated. Two-dimensional correlation spectroscopy (2D-COS) and excitation–emission matrix (EEM) –PARAFAC analysis were applied to investigate the binding affinity and mechanism of BB WSOM. The results showed that pH was a sensitive factor affecting binding affinities of WSOM, and BB WSOMs were more susceptible to bind with Cu(II) at pH 6.0 than pH 4.5, followed by pH 3.0. Therefore, the Cu(II)-binding behaviors of BB WSOMs at pH 6.0 were then investigated in this study. The 2D-absorption-COS revealed that the preferential binding with Cu(II) was in the order short and long wavelengths (237–239 nm and 307–309 nm) > moderate wavelength (267–269 nm). The 2D-synchronous fluorescence-COS results suggested that protein-like substances generally exhibited a higher susceptibility and preferential interaction with Cu(II) than fulvic-like substances. EEM–PARAFAC analysis demonstrated that protein-like (C1) substances had a greater complexation ability than fulvic-like (C2) and humic-like (C3) substances for both BB WSOM. This indicated that protein-like substances within WSOM played dominant roles in the interaction with Cu(II). As a comparison, RS WSOM generally showed stronger complexation capacity than CS WSOM although they exhibited similar chemical properties and compositions. This suggested the occurrence of heterogeneous active metal-binding sites even within similar chromophores for different WSOM. The results enhanced our understanding of binding behaviors of BB WSOM with Cu(II) in relevant atmospheric environments.

Humic acid (HA) is well known as an inexpensive and effective adsorbent for heavy metal ions. However, the thermodynamics of uranium (U) adsorption onto HA is not fully understood. This study aimed to understand the kinetics and isotherms of U(VI) adsorption onto HA under different temperatures from acidic water. A leonardite-derived HA was characterized for its ash content, elemental compositions, and acidic functional groups, and used for the removal of U (VI) from acidic aqueous solutions via batch experiments at initial concentrations of 0&ndash, 100 mg&middot, L&minus, 1 at 298, 308 and 318 K. ICP-MS was used to determine the U(VI) concentrations in solutions before and after reacting with the HA. The rate and capacity of HA adsorbing U(VI) increased with the temperature. Adsorption kinetic data was best fitted to the pseudo second-order model. This, together with FTIR spectra, indicated a chemisorption of U(VI) by HA. Equilibrium adsorption data was best fitted to the Langmuir and Temkin models. Thermodynamic parameters such as equilibrium constant (K0), standard Gibbs free energy (&Delta, G0), standard enthalpy change (&Delta, H0), and standard entropy change (&Delta, S0), indicated that U(VI) adsorption onto HA was endothermic and spontaneous. The co-existence of cations (Cu2+, Co2+, Cd2+ and Pb2+) and anions (HPO42&minus, and SO42&minus, ) reduced U(VI) adsorption. The high propensity and capacity of leonardite-derived HA adsorbing U(VI) suggests that it has the potential for cost-effective removal of U(VI) from acidic contaminated waters.

Adsorption is an important mechanism to immobilize cadmium (Cd) in soil, for which humic substances have a potential. However, commercial humic substances are either very acidic (pH = 2) or alkaline/Na+-enriched, making them less suitable for use in acid and saline soils. Here, we used leonardite to produce humic adsorbents HA (pH = 4.02), Ca-HA (pH = 10.9), and Ca-CPAM-HA (pH = 9.62) by using HCl, CaCl2, or CaCl2-polyacrylamide as a flocculant. Their elemental compositions, acidity, and spectroscopic properties were determined, and their Cd adsorption characteristics were assessed by batch kinetic and thermodynamic experiments at environmentally relevant concentrations. Further, HA was mixed with Cd-contaminated soils and incubated for a month to assess its effect on Cd immobilization. Good fitting of kinetic adsorption data into pseudo-second-order model, together with FTIR spectroscopic data, suggested the chemisorption mechanism by forming Cd(II)-carboxyl complexes. The maximum adsorption capacity derived from the Langmuir equation was 129, 114, and 110 mg Cd(II)/g for HA, Ca-HA, and Ca-CPAM-HA, respectively. These values are almost the same on carbon-normalized basis. HA reduced acetic acid extractable Cd by 31% or more. Besides their high propensity for Cd adsorption, humic adsorbents are inexpensive, safe, and beneficial to soil quality.

With abundant oxygen-containing functional groups, a humic substance (HS) has a high potential to remediate soils contaminated by heavy metals. Here, HS was first extracted from a leonardite and analyzed for its chemical compositions and spectroscopic characteristics. Then it was assessed for its ability as a washing agent to remove Cd and As from three types of soils (red soil, black soil, and fluvo-aquic soil) that were spiked with those contaminants (Cd: 40.5&ndash, 49.1 mg/kg, As: 451&ndash, 584 mg/kg). The operational washing conditions, including the pH and concentration of the HS, washing time and cycles, and liquid&ndash, soil ratio, were assessed for Cd and As removal efficiency. At pH 7, with an HS concentration (3672 mg C/L) higher than its critical micelle concentration and a liquid&ndash, soil ratio of 30, a single washing for 6&ndash, 12 h removed 41.9 mg Cd/kg and 199.3 mg As/kg from red soil, 33.5 mg Cd/kg and 291.5 mg As/kg from black soil, and 30.4 mg Cd/kg and 325.5 mg As/kg from fluvo-aquic soil. The removal of Cd and As from the contaminated soils involved the complexation of Cd and As with the carboxyl and phenolic groups of HS. Outcomes from this research could be used to develop a tailor-made HS washing agent for the remediation of Cd- and As-contaminated soils with different properties.

Anthropogenic activities, such as ore mining and processing, nuclear power generation, and weapon tests, have generated uranium (U) contamination to soils and waters. The mobility and bioavailability of U are influenced by its sources, speciation, and plant species. Phytoremediation has emerged as an environmentally friendly, cost-effective green technology to remediate radioisotope- and metal-contaminated soils. The main objective of this study was to explore the feasibility using sunflower (Helianthus annuus) and Indian mustard (Brassica juncea) in cleaning up soils with UO2, UO3, and UO2(NO3)2. Uranium was found to be bioaccumulated in plant roots more than plant shoots. Uranium uptake by both plant species was significantly higher from the UO3- and uranyl-contaminated soils than from UO2-contaminated soils. UO3- and UO2(NO3)2-contaminated soils showed higher exchangeable, weak acid extractable, and labile U than the UO2-contaminated soils. After a growing season, three U forms decreased as redistribution/transformation of U resulted in U species with lower extractability. This study indicates the importance of U speciation in soil with regard to the potential use of sunflower and Indian mustard for phytoremediation of U-contaminated soils.

The occurrence of uranium (U) and depleted uranium (DU)-contaminated wastes from anthropogenic activities is an important environmental problem. Insoluble humic acid derived from leonardite (L-HA) was investigated as a potential adsorbent for immobilizing U in the environment. The effect of initial pH, contact time, U concentration, and temperature on U(VI) adsorption onto L-HA was assessed. The U(VI) adsorption was pH-dependent and achieved equilibrium in 2 h. It could be well described with pseudo-second-order model, indicating that U(VI) adsorption onto L-HA involved chemisorption. The U(VI) adsorption mass increased with increasing temperature with maximum adsorption capacities of 91, 112 and 120 mg g−1 at 298, 308 and 318 K, respectively. The adsorption reaction was spontaneous and endothermic. We explored the processes of U(VI) desorption from the L-HA-U complex through batch desorption experiments in 1 mM NaNO3 and in artificial seawater. The desorption process could be well described by pseudo-first-order model and reached equilibrium in 3 h. L-HA possessed a high propensity to adsorb U(VI). Once adsorbed, the release of U(VI) from L-HA-U complex was minimal in both 1 mM NaNO3and artificial seawater (0.06% and 0.40%, respectively). Being abundant, inexpensive, and safe, L-HA has good potential for use as a U adsorbent from aqueous solution or immobilizing U in soils.

Cost-effective and eco-friendly washing agents are in demand for Cd contaminated soils. Here, we used leonardite-derived humic substances to wash different types of Cd-contaminated soils, namely, a silty loam (Soil 1), a silty clay loam (Soil 2), and a sandy loam (Soil 3). Washing conditions were investigated for their effects on Cd removal efficiency. Cadmium removal was enhanced by a high humic substance concentration, long washing time, near neutral pH, and large solution/soil ratio. Based on the tradeoff between efficiency and cost, an optimum working condition was established as follows: humic substance concentration (3150 mg C/L), solution pH (6.0), washing time (2 h) and a washing solution/soil ratio (5). A single washing removed 0.55 mg Cd/kg from Soil 1 (1.33 mg Cd/kg), 2.32 mg Cd/kg from Soil 2 (6.57 mg Cd/kg), and 1.97 mg Cd/kg from Soil 3 (2.63 mg Cd/kg). Cd in effluents was effectively treated by adding a small dose of calcium hydroxide, reducing its concentration below the discharge limit of 0.1 mg/L in China. Being cost-effective and safe, humic substances have a great potential to replace common washing agents for the remediation of Cd-contaminated soils. Besides being environmentally benign, humic substances can improve soil physical, chemical, and biological properties.