Description

Introduction

It is estimated that by 2050 the world’s population will reach 9.8 billion out of which 68% will inhabit cities [1][2].

Housing this growing population is a major concern especially since key resources like sand needed for construction are limited. For construction, concrete is essential. The concrete used for construction normally has a ratio is 1:2:4 cement, sand, and aggregates respectively [3].

Sand is one of the most used resources on the earth after water [4].

It ranges from construction to the production of glass, from paving roads to the electronics made. Even though sand is common in deserts, it's scarce in most parts because it's over-extracted for industrial purposes. Growing demand for sand from construction also creates a pressing need to reassess how we source this critical commodity. That is because certain types of sand are more suitable for construction than others, but extracting it from its natural sources, such as rivers and coastal areas, has caused massive damage to the environment, alerting everyone to the long-term viability of sourcing this 'resource' sustainably.

The Problem

Rising temperatures? Check.
Melting ice caps? Check.
Global warming? Check.
Illegal river sand mining leaving scars? Check.

Sand is crucial for the construction industry. Sand is the foundation of human construction and a fundamental ingredient in concrete, asphalt, glass and other building materials [4].

The shortage of sand has led to illegal mining which is devastating to the environment. It is often obtained from dredging riverbeds and coastlines. The illegal mining of sand is another issue gaining momentum in recent times.

Illegal sand mining leaves rivers scarred, depleting ecosystems, threatening the delicate balance of the life they sustain, by the whopping billions habituating Mother Earth.

Sand mining from rivers and marine ecosystems "can lead to erosion, salination of aquifers, loss of protection against storm surges and impacts on biodiversity, which pose a threat to livelihoods through, among other things, water supply, food production, fisheries, or to the tourism industry," says UNEP [2].

Impact

The 50 billion tonnes of sand thought to be extracted for construction every year is enough to build a nine-story wall around the planet [5].

The report builds on UNEP research from 2019 that found increasing demand for sand, which saw a three-fold growth over 20 years, had caused river pollution and flooding, while also shrinking aquifers and deepening droughts [4].

Fig. 1: The environmental impacts of Sand Mining [2]

Fig. 1: The environmental impacts of Sand Mining [2]


Climate-related disasters are now being connected to sand mining. “Keeping sand in the rivers is the best adaptation to climate change,” the WWF’s Marc Goichot told the Thomson Reuters Foundation. “If a river delta receives enough sediment, it builds itself above sea level in a natural reaction.” [2].

Officials found that a whopping 2.59 lakh tonnes of sand was illegally sourced from Koppal district in 2023-24. Kalaburagi: Illegal sand mining cases in north Karnataka, which have devastated rivers, have gone up from 4,402 in 2022-23 to 5,441 in 2023-24 [6].

A 2022 report from UNEP, titled 'Sand and Sustainability: 10 Strategic Recommendations to Avert a Crisis', found that sand extraction is rising about 6 percent annually, a rate which it is unsustainable [7].

How is Sand useful to us?

There are 4 major types of sand being used in construction namely; riverbed sand, marine sand, manufacturing sand, and lastly desert sand. Desert sand is eroded largely by wind. The grains are largely smooth, rounded, and poorly graded. Due to these reasons desert sand is not preferred for construction. It is being currently used as a fine aggregate to fill the void spaces in concrete along with slag, fly ash, and crushed stones.

Recently, efforts have been made to incorporate treated desert sand for construction. Mechanical rolling of grains using high-pressure rollers has been done to create fine-grain sand for it to be used as a fine aggregate.

Shape: Desert sand grains are typically smooth and rounded due to constant wind erosion. This rounded shape prevents them from interlocking effectively with each other, which is crucial for the strength of construction materials like concrete.

Interlocking: Construction materials rely on the interlocking of grains to provide stability and strength. Rounded grains have less surface area for contact, leading to weaker bonds.

Binding: Traditional binders like cement and mortar depend on the interlocking of grains to create a strong structure. Rounded grains don't interlock as well, reducing the effectiveness of these binders.

Our Solution

Recycling construction material from demolition sites and developing the potential of ore-sand are two simple ways to reduce the consumption of new sand while contributing to global circular economy ambitions, the Sand and Sustainability report found [4]. This, of course, is the preliminary part.

We can't keep depleting riverbeds by mining sand. That's why we've developed a sustainable solution: the world is already covered in sand—specifically, desert sand.

Desert sand is largely useless to us. Desert sand grains are the wrong shape. Eroded by wind rather than water, they are too smooth and rounded to lock together to be used for construction. But what if we can engineer bacteria to produce some adhesions, some Exopolysaccharides, which can help the sand bind well together so that we no longer have to excavate river beds for this? Biopolymers as binding material.

Polynucleotides, polypeptides, and polysaccharides are the three main groups of biopolymers, which are organic polymers made by living organisms. The most prevalent biopolymer employed in a variety of fields, including civil and construction engineering is polysaccharides.

Poly (γ-glutamic acid) (PGA) is an amino acid biopolymer that has recently attracted attention due to its biocompatibility, non-immunogenicity, and biodegradability. In addition, PGA can exist either in the water-soluble salt form in the presence of various cations (Na+, K+, Mg2+, Ca2+ or NH4+), or in the water-insoluble free acid form. Due to these advantages, PGA derivatives and composites have received much attention for use in biomedical applications. Indeed, PGA composites have been developed as antimicrobial complexes, vaccine adjuvants, and cancer therapy materials, and are also used in medical devices and tissue regeneration [8]. Besides biomedical applications, what is yet to be explored is its capability as a binding material in construction to make cement, as we propose to use it.

γ-PGA

γ-PGA is a unique anionic homopolymer comprising of D- and L-glutamic acid units, which are joined together via amide linkages between the α-amino and γ-carboxylic acid groups. The molecular structure of γ-PGA is directly affected by the molecular composition, molecular weight, and order of the isomers. Based on the monomer, γ-PGA can exist as γ-L-PGA (homopolymers containing only L-glutamic acid), γ-D-PGA (homopolymers containing only D-glutamic acid), and γ-DL-PGA (random copolymer containing D- and L-glutamic acid).The structure and molecular composition decides its properties. Being an anionic polymer γ-PGA has various properties such as; edible, strong water solubility and water retention, non-toxicity, high biodegradability, high cation exchange capacity, metal-chelating ability, strong antioxidant, antimicrobial peptides activities, and high resistance to thermal decomposition [9].

At present Bacillus species are being used to synthesize this biopolymer naturally. Bacillus subtilis, a gram positive, rod shaped, endospore forming bacteria is generally used for commercial production of γ-PGA.

The Bacillus species producing γ-PGA can be divided into two types: glutamic acid dependent and glutamic acid independent strains. Most strains such as B. subtilis F-2-01, B. subtilis MR-141, B. subtilis chungkookjang, B. licheniformis ATCC 9945a and B. licheniformis WX-02 are glutamate dependent strains. Few strains like B. subtilis TAM-4 and B. amyloliquefaciens LL3 are glutamate independent [9]. Glutamic acid dependent strains produce higher amounts of PGA on the addition of glutamate to the medium, but the bacteria can produce considerable amounts of PGA even in the absence of glutamate because of the operation of the de-novo pathway of L-Glutamate synthesis [9]. Glutamic acid dependent strains give a much higher yield of γ-PGA compared to their counterparts. However, the higher cost of production and more complex fermentation process continue to be their limitations.

A few milligrams of γ-PGA cost several dollars. Low yield from strains and the high production cost prove to be barriers in its commercialization. These challenges, however can be overcome by genetic modifications, optimization of nutrient media and culturing conditions.

Metabolic pathways and biosynthesis of PGA [10]

γ-PGA synthesis is a ribosome independent process that occurs in two steps. In the first step D and L- Glutamic acid are synthesized. The two units formed join together to form γ-PGA in the second step.

It follows the salvage pathway. L-glutamic acid is either synthesized by the bacteria or can be taken up from the external environment. L-glutamic acid undergoes racemization to produce D-glutamic acid. The two enantiomers produced undergo copolymerization to produce γ-PGA, which is then released by the cell into the media. It was found that Mn2+ ions, play a role in regulating the ratio of (D:L) in γ-PGA biosynthesis.

Fig.2: Flowchart showing the salvage pathway for γ-PGA synthesis [10]

Fig.2: Flowchart showing the salvage pathway for γ-PGA synthesis [10]

Culture conditions [10]

Depending on the strain chosen the media and culture requirements vary. Based on nutrient requirement PGA producing bacteria can be glutamic acid dependent or glutamic acid independent. L-glutamic acid acts as the inducer and is a comparatively costly. In most case it 20–30 g/L of L- Glutamic acid was used. Another key component in the media is citric acid. It was reported that through tricarboxylic acid cycle (TCA), citric acid gets converted to isocitrate and -ketoglutarate which subsequently gets converted to glutamic acid and finally PGA[11].

Other factors such as carbon and nitrogen source, pH, ionic strength, aeration and agitation can affect productivity.

Different strains prefer different carbon sources for γ-PGA production. Glucose and glycerol were found to support PGA production in almost all strains. It was reported that glucose acts as better carbon source for cell growth and is utilized at a faster rate. However adding higher concentrations of glucose to the media can be detrimental, as the cell would produce other products like polysaccharides instead of the required PGA. When a combination of glucose with glycerol was added to the media formulation, the efficiency of γ-PGA production increased.

A low glucose concentration of 10g/L or lower is maintained along with a high glycerol concentration of 80g/L. Glycerol stimulates polyglutamyl synthetase enzyme need for PGA production. It also decreases the molecular weight of PGA and thus a decrease in viscosity of the fermentation broth enabling better PGA production and growth of cells.

References

  1. World population projected to reach 9.8 billion in 2050, and 11.2 billion in 2100 | United Nations
  2. Sand mining: how it impacts the environment and solutions | World Economic Forum
  3. Methods of Proportioning Cement, Sand and Aggregates in Concrete – theconstructor.org
  4. Sand mining: the problem with our dwindling sand reserves
  5. Our use of sand brings us “up against the wall”, says UNEP report
  6. Spurt in illegal sand mining cases, North Karnataka rivers ravaged
  7. Sand and Sustainability: 10 Strategic Recommendations to Avert a Crisis | UNEP - UN Environment Programme
  8. Park, S., Sung, M., Uyama, H., & Han, D. K. (2021). Poly(glutamic acid): Production, composites, and medical applications of the next-generation biopolymer. Progress in Polymer Science, 113, 101341. https://doi.org/10.1016/j.progpolymsci.2020.101341
  9. Danfeng Li,Lizhen Hou, Yaxin Gao, Zhiliang Tian, Bei Fan, Fengzhong Wang, Shuying Li.(2022). Recent Advances in Microbial Synthesis of Poly-γ-Glutamic Acid: A Review. Foods (Basel, Switzerland), 11(5), 739. https://doi.org/10.3390/foods11050739
  10. Ishfaq Nabi Najar, Sayak Das (2015). Poly-glutamic acid (pga) - structure, synthesis, genomic organization and its application: a review. International Journal of Pharmaceutical Sciences and Research, 10.13040/IJPSR.0975-8232.6(6).2258-80
  11. Kunioka M and Goto A: “Biosynthesis of poly (γ-glutamic acid) from L-glutamic acid, citric acid, and ammonium sulfate in Bacillus subtilis IFO3335,” Appl. Microbiol. Biotechnol. 1994, 40: 867-872