Homopiperazine grafted mesoporous silicas from rice husk ash for CO2 adsorption.

Chloro-functionalized mesoporous MCM-41, SBA-15, MCM-48 and KIT-6 were synthesized by co-condensation of 3-chloropropyl-trimethoxy-silane (CPTMS) and rice husk ash sodium silicate solution, which is subsequently grafted with a heterocyclic amine, homopiperazine (HPZ). X-ray powder diffraction and BET analysis of the chloro-functionalized mesoporous silicas confirmed the similarity between their structural properties and those obtained from conventional silica sources. CO2 adsorption studies of all HPZ-grafted mesoporous silicas exhibited 8-10 wt% of adsorption capacity and are found to be selective, recyclable and thermally stable. Here, the CO2 adsorption reaction is via the traditional carbamate mechanism. The presence of both secondary and tertiary amine in HPZ influences the high CO2 adsorption capacity. Hence, these HPZ-grafted mesoporous silicas could contribute to CO2 capture as a green, tunable, selective and efficient sorbent.

Arsenic concentration in porewater of an alkaline coal ash disposal site: Roles of siderite precipitation/dissolution and soil cover.

The geochemical behavior of As in porewaters of an alkaline coal ash disposal site was investigated using multilevel samplers. The disposal site was in operation from 1983 until 1994 and was covered with 0.3-0.5m thick soils in 2001 when this study was initiated. Sequential extraction analyses and batch leaching experiments were also performed using the coal ash samples collected from the disposal site. The results suggest the important roles of siderite (FeCO(3)) precipitation/dissolution and soil cover, which have been ignored previously. Arsenic levels in the porewater were very low (average of 10microgL(-1)) when the site was covered with soil due to coprecipitation with siderite. The soil cover enabled the creation of anoxic conditions, which raised the Fe concentration by the reductive dissolution of Fe-(hydr)oxides. Because of the high alkalinity generated from the alkaline coal ash, even a small increase in the Fe concentration (0.66mgL(-1) on average) could cause siderite precipitation. When the soil cover was removed, however, an oxidizing condition was created and triggered the precipitation of dissolved Fe as (hydr)oxides. As a result, the dissolution of previously precipitated As-rich siderite caused higher As concentration in the porewater (average of 345microgL(-1)).