ANALYSIS AND REMOVAL OF ARSENIC FROM NATURAL-GAS USING POTASSIUM PEROXYDISULFATE AND POLYSULFIDE ABSORBENTS

Natural gas from the large Abo gas field in southeastern New Mexico co ntains arsenic (0.2 to 2.5 mug/L) in the form of trialkylarsines. A me thod of analysis for arsenic which is based on the extraction of the a lkylarsines into a saturated solution of potassium peroxydisulfate (PP DS) followed by determination of total arsenic, as AsO4(3-), by hydrid e generation-d.c. helium plasma emission spectrometry (HG-DCHPES) is d escribed. The extraction of the alkylarsines was carried out by either shaking IL gas samples with PPDS or sparging the gas in PPDS solution s. Analytical results of acceptable precision can be obtained using ei ther procedure. Temperature (24-45-degrees-C) or pH (1.10-4.10) had li ttle effect on the As concentrations obtained by the sparging method. The measured arsenic concentrations of natural gas samples stored in C r-Mo steel cylinders decrease with time of storage (approximately 30% in the first 4 weeks). A standard blend of trimethylarsine in CH4 (1 m ole TMA in 10(6) moles of CH4), stored in nickel-containing steel cyli nder, showed a similar decrease in As concentration with times. The me asured arsenic concentrations increase when the temperature of a cylin der of natural gas is heated indicating that part of the alkylarsines may be present in a condensed or physically adsorbed form on the inner walls of the cylinder. Scanning electron microscopy with X-ray microa nalysis of the metal powders (Ni, Cr, Mo, Fe) contacted with TMA indic ate that arsenic-rich solids also containing C and O are formed in the case of Ni and Cr. This phenomenon may represent an additional factor in the observed decrease in As concentration with time of storage. Po lysulfides in solution form (Na2Sx) or solid (Na2S4) can quantitativel y remove the trialkylarsines from natural gas.