Self-regulated immobilization behavior of multiple heavy metals via zeolitization towards a novel hydrothermal technology for soil remediation.

More efficient soil remediation technologies are highly anticipated to treat large quantities of heavy metal-polluted urban sites nowadays. Herein, a novel hydrothermal technology of converting heavy metal-polluted soils into zeolites for in-situ immobilizing heavy metals was proposed. The zeolites (analcime and cancrinite) could be synthesized hydrothermally with certain Na/Si and Al/Si ratios. The formed zeolites could manage to change their species and structure during zeolitization to accommodate different heavy metals in soil according to their size and charge. Since smaller-size Cu2+ was introduced, analcime and some cancrinite possessing small cages could be formed adaptively to immobilize the Cu2+ by replacing Na+ and forming Cu2+-OH and Cu2+-O. Whereas, cancrinite with large channels managed to form to immobilize the larger-size Cd2+ by forming Cd2+-O. Interplanar spacing variation of zeolites also corresponded to their structural change for accommodating different heavy metals. Leaching results showed the amounts of Cu and Cd leached from the synthesized zeolites were reduced to 0.005% and 0.05% respectively, reflecting a more stable immobilization of smaller heavy metals by small cages, in agreement with the results of distribution coefficient (Kd). Negligible effect of pH environment on the leaching rates further confirmed the stable structural immobilization of heavy metals by zeolites.

Ni and Zn/ZnO Synergistically Catalyzed Reduction of Bicarbonate into Formate with Water Splitting.

Conversion of CO2 into value-added chemicals with a facile hydrogen source such as water is always of great interest for sustainable development. In this work, a simple and efficient method of reduction of bicarbonate to formate on a simple Ni powder catalyst with water as the facile hydrogen source and Zn as the regenerable reductant is proposed. The Ni catalyst and in situ formed Zn/ZnO exhibited a synergetic catalytic activity in the conversion of bicarbonate into formate, and a good formate yield of 81% was obtained. Detailed studies revealed that the synergetic catalytic activity between Ni and the in situ formed Zn/ZnO was mainly attributed to (i) the inhibited oxidation of Zn by Ni, leading to more interface of Zn/ZnO; (ii) the decreased growth of ZnO crystal along the [0001] direction, and thus increasing the more polar (0001) Zn face and the (0001̅) O face, which have high activity; and (iii) the enhanced generation of more oxygen vacancies at the Zn/ZnO interface to promote the formate yield. This research demonstrates an efficient method of using a simple and nonprecious metal catalyst for the CO2 reduction into value-added chemicals and provides a better understanding of the synergistic catalytic mechanism of Ni and Zn/ZnO.

Reduction of CO2 with H2S in a simulated deep-sea hydrothermal vent system.

H2S is considered to be an important reductant in abiotic CO2 reduction to organics, however, almost no experimental support has been reported. Herein, the first observation of CO2 reduction to formate with H2S under alkaline hydrothermal conditions is reported, and water is found to act as a hydrogen donor.

A novel hydrothermal method to convert incineration ash into pollucite for the immobilization of a simulant radioactive cesium.

The Fukushima nuclear accident in Japan on March 11, 2011 produced huge amounts of Cs-polluted incineration ashes; conventional solidification methods seem unsuitable for the treatment of large amounts of Cs-polluted ashes. A novel hydrothermal method was developed to directly convert Cs-polluted incineration ash (rice husk ash) into pollucite to immobilize Cs in its crystal structure in situ. Results revealed that pollucite could be synthesized readily over a wide range of added Cs (Cs/Si=0.2-0.6); the addition of more Cs (Cs/Si≥0.5) caused the formation of a small amount of cesium aluminosilicate (CsAlSiO4), which exhibits poor immobilization behavior for Cs. Pollucite could be formed even for a short curing time (1h) or at a low curing temperature (150°C). However, a high curing temperature or a long curing time favored the formation of a pure pollucite. With the added calcium hydroxide, a tough specimen with a flexural strength of approximately 22MPa could be obtained, which suggested that this technology may be applied directly to the solidification of Cs-polluted incineration ashes. Hydrogarnet and tobermorite formations enhanced the strength of the solidified specimens, and meanwhile the formed pollucite was present in a matrix steadily. Leaching test demonstrated that the amount of Cs that leached from the synthesized specimens was very low (0.49×10(-5)-2.31×10(-5)) and even lower than that from the reference hollandite-rich synroc (2.0×10(-2)), although a higher content of Cs was found in the synthesized pollucite specimens (6.0-31.7%) than in the reference (3.7%). Therefore, the hydrothermal conversion of Cs-polluted incineration ash into pollucite can be applied to immobilize Cs directly.

Hydrothermal solidification behavior of municipal solid waste incineration bottom ash without any additives.

Municipal solid waste incineration (MSWI) bottom ash could be solidified with and without slaked lime (calcium hydroxide) addition by a hydrothermal method under steam pressure of 1.56 MPa at 200 °C for up to 72 h. Experimental results showed that CSH gel or tobermorite exerted a main influence on strength development, and without any additives CSH gel was easy to form, while slaked lime addition favored to form tobermorite. Tobermorite seemed to exert a larger effect on the strength development than CSH gel. Leaching results showed that the concentrations of heavy metals dissolved from the solidified specimens were effectively reduced after hydrothermal processing. The immobilization was mainly due to the tobermorite or CSH gel formation, and Pb2+ and Zn2+ seemed to be fixed more readily than Cr6+, which might be the reason that the structural Ca2+ within tobermorite or CSH gel was exchanged by Pb2+ and Zn2+ more easily than Cr6+. In addition, there existed a close relationship between leaching concentration and strength enhancement, and a higher strength seemed to exert a larger effect on immobilization of heavy metals.

Reduction of formic acid to methanol under hydrothermal conditions in the presence of Cu and Zn.

Formic acid is the main breakdown product of mild hydrothermal treatment of carbohydrates. Further conversion to methanol was achieved using Cu as catalyst and Zn as reductant under hydrothermal conditions of 250-325 °C for 3-12 h. Both Cu and Zn used were commercial Cu and Zn powders with particle sizes of 200 mesh. A methanol yield of 32% was achieved at 300 °C for 5h with 6.5 mmol of Cu, 12 mmol of Zn and a filling rate of 44%. Thus, this process may provide a promising solution to producing methanol from biomass by converting carbohydrates into formic acid.

Hydrothermal solidification of municipal solid waste incineration bottom ash with slag addition.

Hydrothermal solidification of municipal solid waste incineration (MSWI) bottom ash has been carried out under saturated steam pressure (1.56 MPa) at 200 degrees C for up to 24 h by mixing quartz, slaked lime and water-cooled blast furnace slag (WBFS). The strength enhancement for the WBFS addition was best. The strength development was shown to be due mainly to tobermorite formation, and the tobermorite formation densified matrix, thus promoting the strength development. WBFS seemed to have a higher reactivity than the quartz during the initial hydrothermal process, which provided more silica available to harden the solidified specimens. However, a longer curing time (24 h) was favorable to the quartz dissolution for tobermorite formation, which in turn, enhanced the strength for quartz addition. Curing time affected the crystal morphology evolution, and the stubby plate of tobermorite seemed to result in a high strength enhancement in this study. Laboratory leaching tests were conducted to determine the amount of heavy metals dissolved from the final solidified specimens, and the leaching results showed that after hydrothermal processing the heavy metals dissolved from the solidified specimens were reduced effectively. As such, the hydrothermal processing may have a high potential for recycling/reusing MSWI ash on a large scale.

Potential utilization of riverbed sediments by hydrothermal solidification and its hardening mechanism.

Hydrothermal solidification of riverbed sediments (silt) has been carried out in a Teflon (PTFE) lined stainless steel hydrothermal apparatus, under saturated steam pressure at 343-473 K for 2-24 h by calcium hydrate introduction. Tobermorite was shown to be the most important strength-producing constituent of the solidified silt. A longer curing time or a higher curing temperature was shown to be favorable to the tobermorite formation, thus promoting strength development; however, overlong curing time (24 h) seemed to affect the strength development negatively. The hardening mechanism consisted of the crystal growth/morphology evolution during the hydrothermal process. The species dissolved from the silt were precipitated first as fine particles, and then some of the particles seemed to build up the rudimental morphology of calcium silicate hydrate (CSH) gel. The CSH gel, with precipitated particles, appeared to cause some reorganization within the matrix, which made the matrix denser and thus gave an initial strength development. Tobermorite, transformed inevitably from the CSH gel, reinforced the matrix with its interlocked structure, and thus further promoted the strength development.

Municipal incineration bottom ash treatment using hydrothermal solidification.

Solidification of municipal incineration bottom ash (MIBA) has been carried out using a hydrothermal processing method, in which the MIBA was first compacted in a mold at 5-20 MPa, and then hydrothermally cured in an autoclave under saturated steam pressure at 150-250 degrees C for 10-72 h. Experimental results showed that the tensile strength of the solidified body was greatly influenced by the addition of NaOH solution and fresh cement in the MIBA. The hydrothermal curing temperature and time exerted a significant influence on the development of tensile strength of solidified body. The strength development is speculated to be due primarily to the formation of 1.1 nm tobermorite. Laboratory leaching tests were conducted to determine the amount of heavy metals dissolved from the solidified bodies and the results showed that under the hydrothermal conditions of this study the leaching of heavy metals was very low. As such, the hydrothermal processing method may have a high potential for recycling MIBA.