The author takes no pleasure informing the reader of these deadly toxins insofar as agarocrete from which they leach is exploited by reef-adjacent coral culturists. No matter what career they have chosen, most people that live in remote tropical regions have difficulty feeding their families. Their business operations must therefore be parsimonious for which they have few resources to change. Collecting and culturing for the reef aquarium hobby is comparatively lucrative so they view the reef as a resource that can support their families for many generations, and as such, conservation becomes self-regulating. Hobbyists should not shy away from purchasing cultured corals because one less wild specimen is removed from the reef each time one is sold. It may be possible to carefully detach them from their agarocrete bases inasmuch as limitless dilution lessens the impacts of their leachates in the wild. The findings of the author’s experiments challenge the current understanding that the microporosity and nitrogen-cycling competency of agarocrete, is comparable to that of natural reef substratum. Most appears unsupportive of meaningful denitrification (Aslett 2024).

The flat agarocrete bases of cultured corals have sizeable underside dimples designed to reduce mass during shipment. Conventionally grown on wire mesh tables in lagoons, when positioned on an even surface in a recirculating system, they entrap standing water which stagnates. Microbial anaerobic fermentation thus generates deadly hydrogen sulphide gas (H₂S) and bisulphide ions (HS^–) which usually “crash” life support, so ensure that all system water can mix and circulate freely.

Gunite, shotcrete, and agarocrete are widely used in public exhibits and domestic aquaria, yet they contain the exceedingly toxic and carcinogenic species, hexavalent (Cr VI) and trivalent chromium (Cr III). The movie Erin Brockovich raised public awareness of the dangers of Cr VI in 2000, where trivalent is converted to hexavalent during the manufacture of cement (CSTEE 2002). To protect people in the UK, reducing agents must maintain soluble Cr VI below 0.2 mg kg^-1 (ppm; LaFarge Cement UK 2009). However, remarkable increments in the bioavailability of Cr VI occur after these agents expire, where 6 mg kg^-1 leached throughout laboratory tests recreating average terrestrial environments (Estokova et al. 2018). Toxic metals are bound as calcium/silicate precipitates within cured matrices which are liberated between pH 8 and 6, where the abundance of Cr VI surpasses that of all harmful metals (Halim et al. 2004). Cements were not intended for submersion while the pH within agarocrete conceivably falls below neutral.

The toxicity of Cr III is evident from 10 µg l^-1 (ppb; 0.01 mg l^-1; ppm) while 0.9 mg l^-1 was lethal to 50 percent of marine fish after 96 hours. 19.3, 10.3, and 100 mg l^-1 killed 50 percent of crustaceans, molluscs, and annelids within 48 hours, whereas 2 mg l^-1 Cr III inhibited 50 percent of “algal” photosynthesis after five days (Australian Government Initiative 2000).

Seawater alkalinity lessens Cr VI lethality to ~4 µg l^-1 (ppb; 0.004 mg l^-1) where teleostean LC₅₀ was 14.1 and 0.78 mg l^-1 at 14 and 21 days, whereas crustacean seven- and 20-day data were 3.1 and 0.004 mg l^-1. Seven-day echinoderm and mollusc LC_50s were 2 and 1.6 mg l^-1, whilst annelid was 2 after a week and 0.025 for a fortnight. Growth inhibition was observed in dinoflagellates and cyanobacteria at an effective concentration 50 (EC₅₀) of 5 µg l^-1 (0.005 mg l^-1) from day seven, while a “no observed effect level” (NOEL) was measured at 1 mg l^-1 Cr VI for diatoms (Australian Government Initiative 2000). See Part I.

Marine fish actively accumulate Cr VI but not Cr III (Sherwood & Wright 1976; van Weerelt et al. 1984) where species of the former are found in marine sediments at 1,500 mg kg^-1 (Sherwood & Wright 1976), albeit in all probability, as bio-unavailable insoluble sulphides (Shimek 2002). Therefore, install deep substrate on all horizontal surfaces that are not exposed to detritus-bearing water.

Fig 1. A synthetic rock and coral consolidation possibly intended for camouflaging and straining a weir, which exploits flexible mesh made from reef safe polypropylene (PP; Advance MSDS 2016) available from garden centers for fencing-in poultry. Nothing adheres to PP upon which objects may be cable tied, but it is unlikely to successfully embed within mortars. However, fibers of PP blend with cements to form stable renders (Austin & Robins 2005). Ensure water contained within columns and all pipework freely circulates with that of the system.

Cr VI is amphoteric insofar as it manifests in both acidic and basic forms (Bae et al. 2017) which are powerful oxidants of intracellular components and compounds. Thermodynamics suggests Cr III and Cr VI will manifest as Cr(H₂O)₄(OH)₂⁺ and CrO₄^2- in seawater; however, biologically unavailable Cr III predominates (Elderfield 1970). Nevertheless, Cr VI persists as CrO₄^2- at optimal reef pH, whereas acidification causes cycling of HCrO₄^– and H₂CrO₄ (Pettine 2000).

Fig 3. Toxic white Portland cement available from UK builder’s merchants.

0.1 mg kg^-1 is the legislative maximum for species of chromium in fish flesh (Bosch et al. 2016) whereas concentrations in seawater range from 10.7 µg l^-1 (0.0107 mg l^-1) to 27 µg l^-1 (0.027 mg l^-1; Geisler & Schmidt 1991). A study of acidic freshwater found that granular activated carbon (GAC) sequesters Cr VI where 45.24 g kg^-1 was absorbed at pH 2 (Yüksel & Orhan 2019); however, seawater evaluations and coral LC_50s are lacking (Seyf-Laye et al. 2010) while GAC may only be used for 48 hours. Water changes and GAC will lessen aqueous Cr, yet diminished diversity is liable to accompany the suffering of chronically-exposed ornamentals.

Fig 4. Agarocrete made from a blend of one part white Portland cement, three parts coral sand, and one and a half of solar salt, whose fissures and holes were created by repeatedly over layering sizeable mounds of salt with mortar. Aslett©.

Hobbyists’ literature affirms cement structures are pH-safe after undergoing a brine cure, which may extract some surface chromium, yet no substantiation claims this alters the bioavailability of Cr III and VI. Cement-comprising live rock agarocrete alternatives are readily available, notwithstanding, sustainably sourced reef-derived, albeit non-reef-bearing, calciferous rock remains the preferred substratum, where harvesting rubble from inhospitable barren reef flats and culturing in shallow sunlit lagoons would minimize impacts.

Solar salt is formed by evaporation and is considered food grade, which comprises toxic impurities at legislative-compliant concentrations. Their accumulation in recirculating systems may thus prove detrimental. Do not use this salt to make synthetic seawater because it is merely a coarse and soluble granulated product to increase the porosity of artificial rock, while agarocrete requires a pH-stabilizing cure in brine for which solar salt is an uncostly source of sodium chloride.

Ostensibly fearmongering non-peer reviewed literature continues to warn against the dangers of “E” numbers, which in some cases are assigned to potentially hazardous and harmful compounds. Solar salt contains the food additive Fe(CN)₆^4- (ferrocyanide; E535) which is approved for use as an anti-caking agent in dietary salt, which does not accumulate in humans. Ferrocyanide successfully decontaminates radioactive sludge (Osmanlioglu 2018), yet alongside its REDOX partner, ferricyanide (Fe(CN)₆^3-) photodecompose to release ionic cyanide (CN^–). Such photodegradation has caused mass kills of wild fish at 2 mg l^-1 (ppm; Burdick & Lipschuetz 2011) hence outcomes are challenging to predict. Manufacturers of table salt add ferrocyanide, and we can assume it may be safely exposed to stomach acid or is considered below its toxic threshold. Ferro- and ferri-cyanide also decompose in the presence of hydrogen peroxide (Girdhar & Jain 1965) while their breakdown and protonation by strong mineral acids liberates the exceedingly lethal gas, hydrogen cyanide (HCN), with the distinctive smell of bitter almonds (Domingo et al. 2011). HCN poisons haemoglobin and invertebrate haemocyanin, yet its predisposition for binding cytochrome oxidase confers its acute lethality which is an indispensable enzyme integral to the respiratory electron transport chains of mitochondria. Hence modified cells cannot use oxygen (Jiang et al. 2016). Our stomach contains the strong mineral acid hydrochloric (HCl), while solar salt may be “safely” exploited in agarocrete moulds and brine cures, whereafter the aquarium and substratum are thoroughly rinsed in copious amounts of R/O water.

Experienced marine aquarists understand that remediation must be protracted, proportional, and nominal. Brine-cured cements have been exploited in tropical marine systems for several decades with “no observed adverse effect”, so if your livestock appear outwardly healthy, then your system has attained, albeit a perhaps precarious, equilibrium.

Join us next time when we discover the truth behind ostensibly reef “safe” plastics from which pumps, wavemakers, protein skimmers, and pipework are fabricated.

References

Advanced MSDS (2016) Safety Data Sheet Polypropylene (PP). S&M-FRM-0007, Rev-6.

Aslett, C., G. (2024) The Complete Reef Aquarist, for hobbyists, the trade, and academics – A Conservation Manual. Aslett, C., G. (ed.). Reef Ranch Publishing Ltd, Leeds, UK. pp 429.

Austin, S. & Robins, P. (2005) A Repair Application for Polypropylene Fibre Reinforced Sprayed Concrete. Fibre Reinforced Cements and Concretes, Recent Developments. Swamy, R., M. & Barr, B. (eds.). Tailor & Francis Group, London. pp 21-31.

Australian Government Initiative (2000) Austria & New Zealand Guidelines for Fresh and Marine Water Quality. Chromium in freshwater and marine water. https://www.waterquality.gov.au/anz-guidelines/guideline-values/default/water-quality-toxicants/toxicants/chromium-2000#:~:text=Factors%20that%20affect%20toxicity%20of,review%20by%20Pawlisz%20et%20al.&text=In%20contrast%2C%20in%20hard%20water,it%20was%2067.4%20mg%2FL

Bae, S., Hikaru, F., Kanematsu, M., Yoshizawa, C., Noguchi, T., Yu, Y. & Ha, J. (2017) Removal of Hexavalent Chromium in Portland Cement Using Ground Granulated Blast-Furnace Slag Powder. Materials. 11(1),.

Bosch, A., O’Neill, B., Sigge, G., Kerwath, S. & Hoffman, L. (2016) Heavy metals in marine fish meat and consumer health: a review. Journal of the Science of Food and Agriculture. 96(1), 32-48.

Burdick, G. & Lipschuetz, M. (2011) Toxicity of Ferro- and Ferricyanide Solutions to Fish, and Determination of the Cause of Mortality. Transactions of the American Fisheries Society. 78,.

CSTEE (2002) Scientific Committee On Toxicity, Ecotoxicity and The Environment (Cstee). Risks to Health From Chromium Vi in Cement. European Commission Directorate-General Health and Consumer Protection. https://ec.europa.eu/health/archive/ph_risk/committees/sct/documents/out158_en.pdf

Domingo, P., Garcia, B. & Leal, J. (2011) Acid-base Behaviour of the Ferricyanide Ion in Perchloric Acid Media Spectrophotometric and Kinetic Study. Canadian Journal of Chemistry. 68, 228-235.

Elderfield, H. (1970) Chromium speciation in sea water. Earth and Planetary Science Letters. 9(1), 10-16.

Estokova, A., Palascakova, L. & Kanuchova, M. (2018) Study on Cr(VI) Leaching from Cement and Cement Composites. International journal of environmental research and public health. 15(4), 824.

Geisler, C., D. & Schmidt, D. (1991) An overview of chromium in the marine environment. Deutsche Hydrographische Zeitschrift. 44, 185-196.

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Halim, C., E., Amal, R., Beydoun, D., Scott, J., A. & Low, G. (2004) Implications of the structure of cementitious wastes containing Pb(II), Cd(II), As(V), and Cr(VI) on the leaching of metals. Cement and Concrete Research. 34(7), 1093-1102.

Jiang, J., Chan, A., Ali, S., Saha, A., Haushalter, K., J., Lam, W., L., Glasheen, M., Parker, J., Brenner, M., Mahon, S., B., Patel, H., H., Ambasudhan, R., Lipton, S., A., Pilz, R., B. & Boss, G., R. (2016) Hydrogen Sulfide–Mechanisms of Toxicity and Development of an Antidote. Scientific reports. 6, 20831.

LaFarge Cement UK (2009) Health and Safety Information Portland cement (BS EN 197-1:CEM I) DAT SHEET. https://simongibsontransport.co.uk/wp-content/uploads/17-Portland-Cement-SDS.pdf

Osmanlioglu, A., E. (2018) Decontamination of radioactive wastewater by two-staged chemical precipitation. Nuclear Engineering and Technology. 50(6), 886-889.

Pettine, M. (2000) Redox Processes of Chromium in Sea Water. Gianguzza, A., Pelizetti, E. & Sammartano, S. (eds.). Chemical Processes in Marine Environments. Environmental Science. Springer, Berlin, Heidelberg. Chapter 15.

Seyf-Laye, A., S., Liu, F. & Chen, H. (2010) Optimization of key parameters for chromium (VI) removal from aqueous solutions using activated charcoal. Journal of Soil Science and Environmental Management. 1, 55-62.

Sherwood, M., J. & Wright, J., L. (1976) Uptake and Effects Of Chromium on Marine Fish. https://ftp.sccwrp.org/pub/download/DOCUMENTS/AnnualReports/1976AnnualReport/ar17.pdf

Shimek R., L. (2002) It’s (In) The Water. Reefkeeping.com http://reefkeeping.com/issues/2002-02/rs/feature/index.htm

van Weerelt, M., Pfeiffer, W., C. & Fiszman, M. (1984) Uptake and release of 51Cr(VI) and 51cr(III) by Barnacles (Balanus sp). Marine Environmental Research. 11(3), 201-211.

Yüksel, Ş. & Orhan, R. (2019) The Removal of Cr(VI) from Aqueous Solution by Activated Carbon Prepared from Apricot, Peach Stone and Almond Shell Mixture in a Fixed-Bed Column. Arab J Sci Eng. 44, 5345-5357.

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