DETOXIFICATION OF NITRITE-CONTAINING AQUEOUS WASTE

摘要

Processes were developed to convert nitrite-containing caustic waste water to neutral and oxidized salt solutions. The product can be used as liquid fertilizer or safely disposed. This compares to current disposal techniques of bulk storage or deep-well injection. The processes developed and described in this paper have applications from coal-fired power plant industry for NO_x control to east/west coast space launch sites where NO_2 is used as hypergolic fuel and controlled via caustic scrubbers. Since aqueous scrubber waste solutions containing nitrites can produce NO_X emissions through chemical pathways promoted by neutral and acidic pH, waste conversion strategies must take into account the sequence of oxidation and neutralization detoxification steps. The detoxification processes outlined here provide the basis for a controlled nitrite-waste treatment scheme which far exceeds the OSHA STEL of 1 ppm. for NO_2.The detoxification processes utilize a liquid-phase oxidation of the scrubber liquor nitrites (NO_(2~-)) to nitrates (NO_(3~-)) followed by neutralization of the hydroxide-nitrate waste stream to a high potassium and phosphate-containing aqueous stream. Two primary treatment schemes were developed: (1) a single-step conversion of nitrite-containing water with hydrogen peroxide and MKP, and (2) a dual-step process with ozone and MKP. Oxidation of nitrites to nitrates with 99+% efficiency were achieved with ozone, and conversions >97% were measured using a hydrogen peroxide treatment process. Upon neutralization of the hydroxide-nitrate waste with monobasic potassium phosphate (MKP), the net conversion of NO_X to neutral aqueous solution was generally greater than 98%. The NO_X emissions measured during the detoxification processes were less than 1 ppm (volume) as monitored using UV/VIS spectrophotometry. The final neutral product contains high percentages of potassium and phosphate, suitable for liquid fertilizer.The processes were developed in bench-scale studies for batch operation using waste solutions containing 0.75 to 7.5 weightpercent nitrite ion, 1.0 to 10 weight percent nitrate ion, and 10 weight percent sodium hydroxide. The processes could be scaled up for the detoxification of stored scrubber waste or used in conjunction with ongoing scrubber operations. The general treatment schemes could also be adapted to the treatment of SO_X scrubber waste, where aqueous sulfites are produced and handled.