Fate of corrosion products released from stainless steel in marine sediments and seawater. Part 1. Northeast Pacific pelagic red clay
To provide information useful for determining the biogeochemical cycling of corrosion products in the benthic boundary layer of the deep ocean, neutron-activated stainless steel was exposed to seawater and to Northeast Pacific red clay under aerobic and non-oxygenated conditions. This report describes the trace metal geochemistry of the sediment and the chemical speciation of the corrosion products . The sediments generally consisted of reddish-brown clay at the surface grading to a dark-brown
... ransition zone below which mottled olive-gray clay prevai 1 ed. Mn, Ni, Cu, and Zn are concentrated in the upper 12 cm of the core. The concentration of extractable Cr is greater in the lower, more reduced section of the sediment. Most of the sediment Cr is not extractable and is likely occluded in the sediment mineral fraction. The distribution of Mn indicates that diffusion and re-oxidation of Mn has occurred following dissolution of Mn oxide in the lower, reduced part of the sediment. Only a small fraction «20%) of the sediment Mn at the surface is not extractable. Up to about 30% of extractable Mn in the upper 6 cm of the sediment was in a form which was complexed readily by DTPA. Most of the Mn (>60%) in the upper section of the core is in the oxidic fraction, and likely served as an adsorbent for ionic trace metals. The amount of extractable Fe in this sediment changed little with depth. Reductant-soluble Co predominated in the upper sediment, but its concentration was greatly diminished in lower sections, indicating that a fraction of Co may have accumulated by processes related to Mn accumulation. The fraction of Ni sorbed to Mn or Fe oxides also decreased with depth indicating that diffusion and adsorption of Ni had occurred. The predomi nance of organi c Cu and the 1 ack of accumul at i on of oxidic Cu near the sediment surface indicates that the complexation of Cu by organic matter was the prevalent means of Cu accumulation. Neutron-irradiated 347 stainless steel specimens were exposed to seawater and sediment slurry under aerobic and non-oxygenated conditions for a period of 108 days. The redox potential measurements for ai r-sparged seawater and sediment slurry, and N 2 , CO 2 -sparged sediment slurry were each 420 mv at the end of the experimental period. The absence of O 2 from the surface sediment at Area W-N apparently will not result in reduced conditions. The presence of aerated sediment more than doubled the amount of corrosion products released compared to aerated seawater and non-oxygenated sediment treatments. The distribution of 60CO released from the stainless steel exposed to aerated seawater show that almost 70% of 60CO activity became associated with suspended particulate matter. The molecular weight distribution of the soluble fraction indicates that about 70% of soluble 60CO activity is either associated with organic molecules of <10 3 nominal molecular weight, forms soluble inorganic ionic pairs, or is in ionic form. Almost 50% of the 60CO activity in particulate matter was in the inorganic or weakly complexed form . Less than 10% was strongly bound to particulate organic matter. About 18% of the 60CO activity in the solids was present in oxides and 21% as refractory particulates. As much as 80% of 60CO activity may have been released to seawater in ionic form and then have undergone adsorption, complexation, oxidation, or reaction with other ions . iii No detectable 60CO activity was present in the soluble, readily dissolved, or inorganic or weakly complexed fractions of aerated sediment which had been used to treat neutron-activated stainless steel. DTPA complexed 10% of total 60CO activity in the sediment, and the largest fractions were associated with reductant-soluble material (32%), likely as oxides, and material soluble in 1:1 HCl (46%) which may be a more refractory oxide. A small fraction (8%) was resistant to the reagents employed. Almost 50% of the 60CO activity was extracted in the combined soluble, easily dissolved, adsorbed, and organically complexed fractions from the non-oxygenated sediment treatment indicating that this much of the corrosion products may be initially released in ionic form. Treatment with a reducing agent removed an additional 23%, a fraction which may also have been released in ionic form and then became strongly associated with sediment oxides. Strong aci d treatment (1: 1 HCl) di sso 1 ved 19% of the 60CO activity and 10% was unaffected by the reagents employed. iv , • ...