MARSci.2002.01.020105

Carbon removal through algal mediated precipitation of calciumcarbonate

Eric J. Mazzone 1,a, Jane L. Guentzel 1,b, and Miguel Olaizola 2

1 Departments of Marine Science and Chemistry, Coastal CarolinaUniversity, P.O. Box 261954, Conway, South Carolina, USA

2 Aquasearch Inc., 73-4460 Queen Ka’ahumanu Highway, Suite 110,Kailua-Kona, Hawaii, 96740

a corresponding author, class of 2003, email: emazz1@yahoo.comb faculty advisor, email: jguentze@coastal.edu

Received 24 June 2002; received in revised form 8 October 2002; accepted 12 October 2002

Abstract

Carbon dioxide emissions have been increasing over the last century, resulting inan increase of atmospheric CO2 concentrations ultimately affecting ocean circulation andglobal climate. Anthropogenic activities have resulted in a 30% increase in the amount ofcarbon dioxide in the atmosphere relative to the pre-industrial concentration of 280p.p.m.v. (Stocker and Schmittner 1997). Industrial emissions provide a large portion ofanthropogenic CO2 released into the atmosphere, and therefore are a target for currentcarbon removal efforts. Although photosynthesis removes atmospheric CO2, thisremoval is not permanent as the CO2 can be re-released to the atmosphere throughbiological processes. The formation of CaCO3 permanently removes CO2 as a solid thatcannot be biologically re-released, thereby making this process a potential tool for theelimination of some anthropogenically produced CO2. This paper discusses experimentsthat were conducted to test the ability of one species of freshwater algae to alter waterchemistry in order to induce the precipitation of solid CaCO3. The estimated rate ofcarbon removal, via CaCO3 precipitate, was 6.6 mM C hr-1.

Keywords: calcium carbonate, carbon removal, global warming, cyanobacteria, carbondioxide

Introduction

Increases in the concentration ofcarbon dioxide (CO2) in the atmosphereare believed to be strongly associatedwith climatic changes and the possiblealteration of the environment in thefuture (Physical Sciences Inc., 2000).With this threat at hand, it is beneficialto investigate methods for minimizinganthropogenic emissions of CO2, asanthropogenic activities have resulted ina 30% increase in the CO2 content of theatmosphere since pre-industrial times(Stocker and Schmittner, 1997). Carbonsequestration involves the capture andstorage of carbon, thereby removing itfrom the global carbon cycle (PhysicalSciences Inc., 2000). Photosynthesis is aprocess whose potential for carbonassimilation is well known. It has alsobeen determined that aquatic microalgaecan use this process to assimilate carbonmore efficiently than land based plants(Physical Sciences Inc., 2000).Unfortunately, this type of carbonstorage is not permanent as algal cellscan respire CO2 and be consumed byother organisms that also respire thecaptured CO2.

Some species of aquaticmicroalgae have the ability topermanently sequester carbon throughthe formation of solid CaCO3. Carbonstored in such a way cannot beconsumed and respired back into theatmosphere, making CaCO3 a valuablesink for permanent carbon sequestration.This process occurs naturally and isknown to produce such occurrences asthe Bahamas whitings (Leal, 1992), andhas furthermore been identified as apossible mechanism for atmosphericcarbon removal in the Earth’s earlyhistory (EPRI, 1993). If this natural

process can be produced commerciallyon a large scale, it may be possible toreduce the amount of anthropogenic CO2released into the atmosphere.

The hypothesis states thatthrough photosynthesis, microalgae alterthe pH of their environment, allowingfor the precipitation of solid CaCO3.Spec i f i ca l ly , a s t he a lgaephotosynthesize, CO2 is removed fromthe environment and the pH of the mediais increased. This increase in pH causesa shift in ion speciation, with thepercentage of CO3

-2 ions in the mediaincreasing as a function of pH (Fig. 1).

The CO3-2 ions bind with Ca2+ ions added

to the media to form solid CaCO3precipitate. With the addition of moreCO2 to the system, the concentration ofthe carbonate ions increases, makingmore CO3

-2 available for precipitation

with Ca2+ ions. This process allows forthe storage of carbon in a solidpermanent form.

Materials and Methods

Experiments were conductedusing a locally isolated, unidentifiedfreshwater species of filamentouscyanobacteria (AQ0012). The cultureswere grown using a freshwaterphotoautotrophic nutrient mediadeveloped by Aquasearch Inc., calledformula 413 (FW 413). The experimentswere conducted using a 14:10 light:dark

cycle with a light intensity of60m E m-2 s-1. The species wasmaintained in the growth phase using a3.3L chemostat system (Fig.2). Thesystem provided a continuous supply ofnutrients and maintained conditionsnecessary for biomass growth. The pH(7.8±0.7) of the chemostat culture wasmaintained via automatic injection ofCO2 in response to changes in pH. Aconstant outflow of media and culturewas collected in a carboy called areceiver. Cultures used for allexperiments were removed from boththe chemostat and receiver.

Standard methods of titrationwere used to determine total alkalinity.A series of algorithms were used inorder to determine total alkalinity, CO3

-2

ion, HCO3- ion, free CO2, and total

inorganic carbon concentrations. Thesealgorithms were based on chemicalspeciation as dictated by thermodynamicequilibrium. A series of equations weredeveloped to calculate total alkalinityand inorganic carbon species using well-established values for the equilibriumconstants and known values of pH andionic strength (Clesceri et al., 1995). Thecalcium species used in all experimentswas CaSO4*2H2O (gypsum) and will be

referred to as Ca throughout this paper.Calcium was added to the media as asaturated solution of gypsum. Thegypsum was fully dissolved in thegrowth media.

When ready to test, the mediawas centrifuged, filtered, and dried in anoven. Concentrated HCl was added tothe dried filtrate and the occurrence of achemical reaction between HCl andCaCO3 was determined through visualinspection. Bubbling of the filtrateindicated that CaCO3 (s) was present.This test was used throughout all of theexperiments to determine the presence ofCaCO3 . The addition of concentratedHCl to solid CaSO4*2H2O did notproduce any visible signs of a chemicalreaction or bubbling. In addition tovisual inspection for CaCO3 thechemical speciation model Mineql+

(Schecher and McAvoy, 2001) was usedto predict the formation of dissolved andsolid species within the media. Themodel calculations predicted that theonly solid species formed in the mediawas CaCO3. Acco rd ing t othermodynamic equilibrium, gypsum didnot reprecipatate in this growth media.

The first experiment wasconducted to determine the removal rateof inorganic carbon from the media as afunction of increasing pH. Theexperiment utilized a control flask andan experimental flask. The reportedresults are an average of 2 trials. Thecontrol flasks were prepared with FW413 media containing existing culture.The experimental flasks were preparedwith the same inoculated media, andthen saturated with Ca. Initial pH andalkalinity measurements were takenfrom all flasks. All flasks were grownunder the previously specifiedconditions. Alkalinity and pHmeasurements were taken periodically.

After the pH of each flask reached 9.0 orhigher, the experiment was terminated.The solids were harvested from theexperimental flask according to themethods described above. CaCO3 (s) wasdetermined as previously stated.

The second experiment wasdesigned to test the feasibility of acontinuously replenished system toprecipitate CaCO3 as a function ofincreasing pH. A chemostat system wasestablished with a culture ofcyanobacteria AQ0012 grown in FW413 media that had been saturated withCa. Initial measurements of alkalinityand pH were taken from the chemostat.Once the culture had grown for 4 days,addition of media and removal of theculture from the chemostat began at aflow rate of 1.97 ml min-1. The pH ofthe chemostat remained relativelyconstant (7.8±0.7). Once a constantsupply of new calcium saturated mediawas initiated to the chemostat, the mediaand culture overflow from the chemostatwere directed to a receiver. The pH ofthe receiver was not controlled. Excessmedia and culture were removed fromthe receiver periodically to preventoverflow. Alkalinity and pHmeasurements were taken daily fromboth the chemostat and receiver cultures.

Results

All of the experiments containingcalcium in the media resulted in whiteparticles observed in suspension amongthe biomass (Fig.3). Results from thefirst experiment with AQ0012demonstrated a significant decrease inthe concentration of total inorganiccarbon from 1.26 mM C to 0.77 mM C(19 % day-1) relative to the control (5%day-1) (F g. 4 & 5). Since no precipitate

was formed in the control, the removalof inorganic carbon was attributed tobiomass incorporation. The whiteamorphous particles found within theexperimental media were determined tobe CaCO3 (S) after being tested forreaction with concentrated HCl. Theestimated rate of carbon removal was

calculated to be 6.6 mM C hr-1 ( Fig. 6).The removal rate of inorganic carbonwas determined by the difference in theconcentration of total inorganic carbonbetween the control media and theexperimental media as a function oftime. The calculations were based on theassumption that the control biomassequaled the experimental biomass sinceboth were incubated under the sameconditions and were inoculated withequal volumes of seeded mediacontaining equal cell densities.

The results from experiment 2indicated that the total inorganic carbonconcentration in the chemostat decreasedfrom 1.3 to 1.0 mM C (1.0 % day –1),while the pH remained relativelyconstant (Fig. 7). The concentration oftotal organic carbon in the receiver,which received no pH control via CO2addition, decreased from 1.74 to 0.54mM C (3.2 % day –1) (Fig.8). Speciationcalculations indicated that theconcentration of CO3

-2 ions increasedfrom 0.2% of the total inorganic carbon

at the beginning of the experiment to 15-33% of the total inorganic carbon at theend of the experiment (Fig. 8). Theparticulate matter was determined to beCaCO3 (s) upon testing with concentratedHCl.

Discussion

E x p e r i m e n t a t i o n w i t hcyanobacteria species AQ0012 yieldedpromising results. The results indicatedthat carbon was removed from thesystem as solid CaCO3. Priorexamination of calcium carbonateformation by cyanobacteria shows thatunder certain conditions, filaments of theorganism can become encrusted withCaCO3 (Merz-Preiss; 2000).Examination of the AQ0012 culture,indicated that particulate CaCO3 was notencrusted on the cells of the organism,however white particles were abundantin close proximity to clumps of the algalfilaments (Fig. 3).

The chemostat experimentyielded results similar to the flaskexperiments in that it produced visibleparticulate CaCO3 in both the chemostatand the receiver at pH values aboveapproximately 8.3. Although we wereunable to quantify the difference, it wasvisually determined that more precipitate

was formed in the receiver than in thechemostat. There was an increase inCO3

-2 ions available for the production ofCaCO3 in both the chemostat andreceiver, but higher pH mediatedcarbonate ion concentrations in thereceiver explain why more precipitatewas formed in this vessel. This datademonstrates that it is possible to have acontinuously replenished system toprecipitate CaCO3 ions as the pH of themedia is increased biologically.

From an industrial perspective,this process has the possibility ofdecreasing carbon emissions that lead toglobal warming. This species ofcyanobacteria has demonstrated itsability to form an environmentconducive to CaCO3 formation, and is apotential candidate for use in large-scalesequestration experiments of anindustrial capacity. As the sequestrationmethod requires a calcium supply, arelatively inexpensive source isCaSO4*2H2O, or gypsum. Deposits ofthis mineral are abundant throughout theworld and it is readily available for usein agriculture as well as other venues.The preceding experiments were allconducted using gypsum as the calciumsource, and this mineral has provensuccessful in its ability to supply calciumto an algal media. The use of thismineral has limited potential due to itsrelatively low solubility. CaSO4*2H2Ois less soluble than other species of Ca,and therefore limits the number of molesof Ca available for binding with freeCO3

-2 ions in the experiments. Anothermore soluble source of Ca could be usedto provide more Ca2+ ions to a medium.Future research should target otherspecies that may increase pH at a fasterrate, allowing a greater rate of solidprecipitate formation in the media. Thisresearch demonstrates the feasibility of

this method, although more informationis necessary to successfully establish anindustrial scale carbon sequestrationsystem. This process may prove to beaf fordable to indus t ry andenvironmentally beneficial. Furtherstudies are needed to determine amethod for the physical separation of thebiomass from the precipitate in order tocollect the solid, as well as a method forquantifying the amount of carbonincorporated into biomass and theinorganic precipitate.

Acknowledgments

This work was supported in partby the ERC Program of the NationalScience Foundation under award numberEEC-9731725 (Aquasearch Inc.), and inpart by the U.S. Department of Energyunder award number DE-FC26-00NT40934 (Aquasearch Inc.). Dr.Valgene Dunham (NSF-AIRE) ofCoastal Carolina University providedtravel funding for E.M. Mazzone and J.L. Guentzel to present this research atscientific meetings. We would like tothank Aquasearch Inc and the MarineBioproducts Engineering Center. Also,thanks to Alex Diffley, Lynn Griswold,Sharlene Naidas, Sara Peck, and JulieThistlethwaite.

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