Removing mercury from aqueous solution using sulfurized biochar and associated mechanisms

Removal of Aquatic Cadmium Ions Using Thiourea Modified Poplar Biochar

Removal of aquatic cadmium ions using biochar is a low-cost method, but the results are usually not satisfactory. Modified biochar, which can be a low-cost and efficient material, is urgently required for Cd-polluted water and soil remediation. Herein, poplar bark (SB) and poplar sawdust (MB) were used as raw materials to prepare modified biochar, which is rich in N- and S- containing groups, i.e., TSBC-600 and TMBC-600, using a co-pyrolysis method with thiourea. The adsorption characteristics of Cd2+ in simulated wastewater were explored. The results indicated that the modification optimized the surface structure of biochar, Cd2+ adsorption process by both TSBC-600 and TMBC-600 was mainly influenced by the initial pH, biochar dosage, and contact time, sthe TSBC-600 showed a higher adsorption capacity compared to TMBC-600 under different conditions. The Langmuir adsorption isotherm model and pseudo-second-order kinetic model were more consistent with the adsorption behavior of TSBC-600 and TMBC-600 to Cd2+, the maximum adsorption capacity of TSBC-600 and TMBC-600 calculated by the Langmuir adsorption isotherm model was 19.998 mg/g and 9.631 mg/g, respectively. The modification method for introducing N and S into biochar by the co-pyrolysis of biomass and thiourea enhanced the removal rate of aquatic cadmium ions by biochar.

Use of hardwood and sulfurized-hardwood biochars as amendments to floodplain soil from South River, VA, USA: Impacts of drying-rewetting on Hg removal

Periodic flooding and drying conditions in floodplains affect the mobility and bioavailability of Hg in aquatic sediments and surrounding soils. Sulfurized materials have been recently proposed as Hg sorbents due to their high affinity to bind Hg, while sulfurizing organic matter may enhance methylmercury (MeHg) production, offsetting the beneficial aspects of these materials. This study evaluated hardwood biochar (OAK) and sulfurized-hardwood biochar (MOAK) as soil amendments for controlling Hg release in a contaminated floodplain soil under conditions representative of periodic flooding and drying in microcosm experiments in three stages: (1) wet biochar amended-systems with river water in an anoxic environment up to 200 d; (2) dry selected reaction vessels in an oxic environment for 90 d; (3) rewet such vessels with river water in an anoxic environment for 90 d. In Stage 1, greater Hg removal (17-98% for unfiltered total Hg (THg) and 47-99% for 0.45-μm THg) and lower MeHg concentrations (<20 ng L^-1) were observed in MOAK-amended systems (10%MOAKs). In Stage 3, release of Hg in 10%MOAKs was eight-fold lower than in soil controls (SedCTRs), while increases in aqueous (up to 21 ng L^-1) and solid (up to 88 ng g^-1) MeHg concentrations were observed. The increases in MeHg corresponded to elevated aqueous concentrations of Mn, Fe, SO₄^2-, and HS^- in Stage 3. Results of S K-edge X-ray absorption near edge structure (XANES) analysis suggest oxidation of S in Stage 2 and formation of polysulfur in Stage 3. Results of pyrosequencing analysis indicate sulfate-reducing bacteria (SRB) became abundant in Stage 3 in 10%MOAKs. The shifts in biogeochemical conditions in 10%MOAKs in Stage 3 may increase the bioavailability of Hg to methylating bacteria. The results suggest limited impacts on Hg removal during drying and rewetting, while changes in biogeochemical conditions may affect MeHg production in sulfurized biochar-amended systems.

Self-engineered iron oxide nanoparticle incorporated on mesoporous biochar derived from textile mill sludge for the removal of an emerging pharmaceutical pollutant

In the present work, low-cost and efficient iron oxide nanoparticle incorporated on mesoporous biochar was prepared from effluent treatment plant (ETP) sludge collected from the textile industry. This sludge contains a higher amount of Fe due to the use of ferric chloride as a coagulant in the treatment of wastewater generated during the process. The raw sludge and prepared biochar was extensively examined by various sophisticated techniques like XRF, XRD, BET, TGA, XPS, RAMAN, FTIR, FESEM, TEM, and VSM. TEM and XRD analysis confirms the presence of iron oxide nanoparticles on mesoporous biochar. The prepared biochar was found to possess BET surface area of 91 m² g^-1. Several parameters like pH, dose, initial concentration, temperature and time were optimized for the adsorptive removal of ofloxacin (OFL) from aqueous solution. Biochar (named as BTSFe) achieved ≈96% removal efficiency of OFL with a maximum adsorption capacity (q_m) of 19.74 mg g^-1 at optimum condition. π-π electron-donor-acceptor and H bonding were the major mechanisms responsible for the OFL adsorption. Kinetic and equilibrium thermodynamic study of showed that the adsorption of OFL was represented by the pseudo-second-order kinetics model, and the process was exothermic and spontaneous. Additionally, Redlich-Peterson and Freundlich isotherms best fitted the experimental data indicating multilayer adsorption phenomenon. Biochar was magnetically separated and thermally regenerated after each cycle for five times with a nominal overall decrease of ≈8% in removal efficiency. Leaching of iron during the adsorption process was also checked and found to be within the permissible limit. This study provides an alternative application of the textile industry sludge as an efficient, low-cost biochar for the removal of emerging pharmaceutical compounds.

Removing tetracycline and Hg(II) with ball-milled magnetic nanobiochar and its potential on polluted irrigation water reclamation

The feasibility of ball-milled magnetic nanobiochars (BMBCs) derived from wheat straw for adsorptive removal of tetracycline (TC) and Hg(II) from aqueous solution was assessed against that of pristine magnetic biochars (PMBCs). Ball milling conversion of PMBCs into BMBCs greatly improved TC and Hg(II) removal, and ≥ 99% TC and Hg(II) were adsorbed by BMBC prepared at 700 °C (BMBC700) within 12 h. The maximum adsorptive removal capacities of BMBC700 for TC and Hg(II) were 268.3 and 127.4 mg/g, respectively. The amounts of TC and Hg(II) removed by BMBC700 decreased gradually as the ionic strength of the solution increased, but increased as the solution temperature increased from 25 to 45 °C. The further FTIR and XPS analysis confirmed removal of TC was predominately regulated by the combination of electrostatic interactions, hydrogen bonds, and Cπ-Cπ interaction, while, the adsorption of Hg(II) was mainly governed by several mechanisms, including electrostatic attractions, Hg-Cπ bond formation, and surface complexation. Overall, BMBC700 presented great potential for TC and Hg(II) removal from polluted irrigation water and exhibited acceptable recyclability performance as well as magnetic separation advantage in use.

Key factors and microscopic mechanisms controlling adsorption of cadmium by surface oxidized and aminated biochars

Modified biochar has great potential for adsorbing cadmium (Cd) in the aquatic environment, but the micro-immobilization mechanisms, driven by surface modifications, remain unclear. There has been no attempt to determine the key adsorption factors by integrating the numerous physiochemical indicators. In this study, surface oxidized biochar (OPBC) and surface aminated biochar (APBC) were prepared from porous biochar (PBC), and the Cd adsorption mechanisms by the modified biochars at the molecular and electronic scales were investigated. The adsorption capacity of APBC and OPBC for Cd was 23.54 and 19.04 mg g^-1, respectively, which was about three times higher than that of PBC. Macroscopically, physicochemical adsorption and intraparticle diffusion dominated the Cd adsorption, and surface properties, such as functional groups, were identified as key factors controlling adsorption. Microscopically, the adsorption of Cd mainly occurred in regions rich in π electrons, lone pair electrons and electron donor groups. The interaction between carboxyl and Cd dominated the adsorption performance of OPBC, while the Cd²⁺-π interaction was weakened by increasing the π electron electrostatic potential of aromatic rings. The lone pair electrons of the amino groups dominated the complexation of APBC with Cd, and the π electron electrostatic potential was almost unaffected.

Impact of biochar on mobilization, methylation, and ethylation of mercury under dynamic redox conditions in a contaminated floodplain soil

Mercury (Hg) is a highly toxic element, which is frequently enriched in flooded soils due to its anthropogenic release. The mobilization of Hg and its species is of ultimate importance since it controls the transfer into the groundwater and plants and finally ends in the food chain, which has large implications on human health. Therefore, the remediation of those contaminated sites is an urgent need to protect humans and the environment. Often, the stabilization of Hg using amendments is a reliable option and biochar is considered a candidate to fulfill this purpose. We tested two different pine cone biochars pyrolyzed at 200 °C or 500 °C, respectively, with a view to decrease the mobilization of total Hg (Hg_t), methylmercury (MeHg), and ethylmercury (EtHg) and/or the formation of MeHg and EtHg in a contaminated floodplain soil (Hg_t: 41 mg/kg). We used a highly sophisticated automated biogeochemical microcosm setup to systematically alter the redox conditions from ~-150 to 300 mV. We continuously monitored the redox potential (E_H) along with pH and determined dissolved organic carbon (DOC), SUVA₂₅₄, chloride (Cl^-), sulfate (SO₄^2-), iron (Fe), and manganese (Mn) to be able to explain the mobilization of Hg and its species. However, the impact of biochar addition on Hg mobilization was limited. We did not observe a significant decrease of Hg_t, MeHg, and EtHg concentrations after treating the soil with the different biochars, presumably because potential binding sites for Hg were occupied by other ions and/or blocked by biofilm. Solubilization of Hg bound to DOC upon flooding of the soils might have occurred which could be an indirect impact of E_H on Hg mobilization. Nevertheless, Hg_t, MeHg, and EtHg in the slurry fluctuated between 0.9 and 52.0 μg/l, 11.1 to 406.0 ng/l, and 2.3 to 20.8 ng/l, respectively, under dynamic redox conditions. Total Hg concentrations were inversely related to the E_H; however, ethylation of Hg was favored at an E_H around 0 mV while methylation was enhanced between -50 and 100 mV. Phospholipid fatty acid profiles suggest that sulfate-reducing bacteria may have been the principal methylators in our experiment. In future, various biochars should be tested to evaluate their potential in decreasing the mobilization of Hg and to impede the formation of MeHg and EtHg under dynamic redox conditions in frequently flooded soils.

Effects of Farming Activities on the Biogeochemistry of Mercury in Rice-Paddy Soil Systems

The biogeochemistry of mercury (Hg) in rice-paddy soil systems raises concerns, given that (1) the redox potential in paddy soil favors Hg methylation and (2) rice plants have a strong ability to accumulate methylmercury (MeHg), making rice an important source for MeHg exposure to humans. Therefore, all factors affecting the behavior of Hg in rice-paddy soils might impact Hg accumulation in rice, with its subsequent potential risks. As a typical wetland, paddy soils are managed by humans and affected by anthropogenic activities, such as agronomic measures, which would impact soil properties and thus Hg biogeochemistry. In this paper, we reviewed recent advances in the effects of farming activities including water management, fertilizer application and rotation on Hg biogeochemistry, trying to elucidate the factors controlling Hg behavior and thus the ecological risks in rice-paddy soil systems. This review might provide new thoughts on Hg remediation and suggest avenues for further studies.