Silicate solubilizing bacteria as an option for concrete remediation.

Concrete is one of the largest components in construction and development waste each year. According to the EPA (2020) concrete accounted for 67.5% of this total waste generation. Modern concrete can take up to 5 decades to decompose in a landfill. The ongoing pressure for sustainable and recyclable products has pushed for a solution that can quickly remediate this concrete waste. Modern concrete is a mixture of water, aggregate, and cement. Cement is the component in concrete that binds the rock or other aggregates together during the curing process. Portland cement, which is the binding component in this study, is composed of gypsum and calcium compounds including tricalcium trisilicate and tetracalcium aluminoferrite (Bye, 1999). Each component is found in varying ratios, but the highest percent compound is tricalcium silicate, which makes up 50% of the binding agent. Bacteria have been identified that can solubilize silicate compounds. These silicate-solubilizing bacteria (SSB) amplify weathering for rocks in nature, and the free silicate is used as a nutrient for the plants. These bacteria have been used in agriculture as a way to supplement the nutrients to rice and wheat. Currently, SSB have been identified in the genera Bacillus, Burkholderia, Janthinobacterium, Aminobacter, and Pseudomonas (Raturi et al., 2021). These genera and other unidentified SSB could help with the issue of concrete waste as it contains high levels of silicates, and it could allow the concrete to replenish nutrients for plant growth in agricultural habitats.

The aim of this study is to identify silicate-solubilizing bacteria and determine the efficacy of the bacteria to break down the cement holding commercial concrete together. A secondary aim is to find an option that is cost effective and environmentally safe. Using the differential medium by Bunt and Rovira I identified several bacteria from our standard lab culture collection that were capable of solubilizing silicate. The mechanism for silicate solubilization is thought to rely on acid production from the bacteria, so the identified bacteria were tested to determine the acid production in glucose-rich culture media. Serratia marcescens was selected as the best candidate from the results of this experiment. Due to the alkaline nature of concrete curing, the bacteria were then tested for survivability in media that contained concrete fragments and powdered concrete. This was completed with absorbance readings and plate counts sampled from the trial. Serratia marcescens was then added to containers with ~5-g fragments of concrete. A dry weight determination was taken before and after each trial as a measure of efficacy. The samples were incubated at room temperature (22–25ºC) and pH was monitored periodically for the two-week trial period. To ensure any change in weight was due to the bacteria and not the media, cell-free controls were run concurrently. The results showed a significantly greater change in weight in trials with bacteria. Based on these results I ran trials that contained bacterial supernatant as well as trials with commercially purchased acids to determine if the presence of the bacteria was necessary. The results showed that the supernatant had no significant change on the concrete. The pure acid had mixed results with acetic acid outperforming the bacterial trials and the others tested being ineffective. These results confirmed that the bacteria are capable of
degrading the concrete faster than in its natural state and gives support to the hypothesis that acid production is important to concrete remediation for SSB.

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