Phase Distribution of Nickel, Chromium, and Cobalt in Seawater at Olivine-Enriched Beaches: Implications for Carbon Capture by Enhanced Silicate Weathering

Author

Nathaniel J. Padrnos, California Polytechnic State University, San Luis ObispoFollow

Available at: https://digitalcommons.calpoly.edu/theses/2955

Date of Award

8-2023

Degree Name

MS in Civil and Environmental Engineering

Department/Program

Civil and Environmental Engineering

College

College of Engineering

Advisor

Yarrow Nelson

Advisor Department

Civil and Environmental Engineering

Advisor College

College of Engineering

Abstract

The application of olivine to coastal areas is a proposed method of removing carbon dioxide from the atmosphere for climate change mitigation. Olivine is abundant and dissolves relatively fast, but it contains trace amounts of nickel (Ni), chromium (Cr), and cobalt (Co) which may be toxic to marine biota in cases of widespread coastal olivine spreading. Prior studies have suggested that Ni concentrations in marine sediments and/or the overlying water column due to olivine dissolution could be a limiting factor for its carbon capture potential. Therefore, it is critical to understand and model the processes affecting trace metal bioavailability after olivine application. This research used sand and sediments collected from a natural olivine beach (Papakōlea) and a black sand control beach (Richardson) in Hawaii to measure the adsorption of Ni, Cr, and Co to suspended particulate matter (SPM) in the water column and to beach sediments in equilibrium with pore water. The experimentally derived adsorption isotherms were then used for phase distribution modeling in the water column and sediment pore water at both sites.

Cr was observed to readily precipitate in seawater without suspended solids, meaning its exposure pathway for marine organisms will likely be via sediment, regardless of interactions with solid phases. Co uptake from solution may have been influenced by biological processes based on its continuous adsorption over 30 days, but more study ofcobalt adsorption to SPM is required because of conflicting results in a repeated experiment. Thus, Ni was the focus of the phase distribution modeling in this work.

Ni adsorption from seawater to SPM generated from both Papakōlea and Richardson sediments followed Langmuir adsorption models. Parameters for Ni adsorption to SPM from Papakōlea at pH 7.81 were KL = 2.74 L/kg and qmax = 931 mg/kg and for Richardson SPM at pH 8.00 they were KL = 5.69 L/kg and qmax = 1446 mg/kg. Ni adsorption to SPM was strongly affected by pH with the qmax for Richardson SPM nearly eight times greater at pH 8.00 than at pH 7.77. Ni adsorption to Papakōlea SPM was five times greater than Ni adsorption to Richardson SPM at comparable pH.

Ni adsorption to beach sediments from both sites was measured for five pHs between 7.27 and 8.73. Ni adsorption was observed to be strongly affected by pH and sediment type. A significant adsorption edge was observed for the Papakōlea sediment between pH 8.00 and 8.22, while a gradual increase in adsorption was observed between pH 7.27 and pH 8.15 for the Richardson sediment.

Ni adsorption to the beach sediments from pore water at the typical seawater pH of 8.05 was calculated by interpolation between adsorption models at surrounding pH values. Calculated Ni adsorption to Papakōlea sediment at pH 8.05 was linear with Kd = 22.4 L/kg while Ni adsorption to Richardson sediment at pH 8.05 followed a Langmuir model with KL = 0.8 L/kg and qmax = 179 mg/kg.

Phase distribution models constructed with the experimentally derived adsorption isotherms predicted that over 85% of the Ni in the water column is dissolved. This result was similar for the water columns at both beaches and was based on typical coastal total suspended solids (TSS) concentrations (5-20 mg/L). Therefore, it is predicted that transport of Ni into marine sediments by SPM settling is much smaller than bulk transport of dissolved Ni into the open ocean.

In contrast, the pore water models predict that over 95% of total Ni is adsorbed to the solid phase for both sediments. In a scenario where dissolved Ni concentration in the pore water is the maximum safe level of 8.2 ppb (US EPA environmental quality standard; EQS), Ni adsorbed to the sediment at both sites is predicted to be less than 10% of safe levels, the Florida Department of Environmental Protection’s threshold exposure limit (TEL) of 15.9 mg Ni per kg of sediment. These adsorbed Ni concentrations are likely a better indicator of threshold sediment Ni concentrations because presumably Ni adsorbed to sediments is more readily bioavailable than Ni in the structure of the olivine. In this case, dissolved Ni would be of more environmental concern than Ni in seabed sediments at sites of olivine enrichment. However, it will be important to investigate the bioavailability of Ni directly from the olivine structure to confirm this conclusion.

If dissolved Ni concentration in the pore water is the limiting factor for safe application of olivine, then olivine loading rates should be based on site-specific conditions (mixing regime and the residence time of seawater in the sediment pores) to maintain a dissolved Ni concentration less than the EQS. For Papakōlea Beach, with 70% olivine in thesediments, the dissolved Ni concentrations in the pore waters were measured to be at background seawater concentrations. This is likely because of the rapid mixing and high- energy wave action at this beach. Under these conditions, very little Ni would be adsorbed to the sediments.

Additional site-specific studies of Ni and Co adsorption to beach sediments should be conducted to gain a more complete understanding of the fate and transport of these trace metals and the risks posed to coastal ecosystems by their release. More experiments are needed to characterize Co adsorption in seawater to SPM and beach sediment. Additionally, Ni adsorption experiments should be performed on a wider variety of beach sediments to estimate its fate and transport at specific locations of potential olivine enrichment. Also, Ni adsorption to Papakōlea SPM was measured at pH 7.81, and since adsorption is expected to be much greater at seawater pH, the Ni adsorption experiment should be repeated at a pH greater than that of seawater to allow the construction of adsorption isotherms for Papakōlea SPM at seawater pH by interpolation. Additionally, experiments need to be conducted to determine the bioavailability of trace metals in the olivine itself and adsorbed to the olivine.