Use of crushed mussel shells for the remediation of heavy metal contaminated soils

Popular Abstract

Can mussel shells cure heavy metal polluted soils?

Heavy metal contamination in soils has become a global concern, as it has recently been increasing due to anthropogenic factors, such as agricultural and mining activities. These practices have led to elevated and unsustainable metal concentrations in soils, causing toxicity and fatality to plants and other living organisms. Heavy metal polluted soils have been characterized by high soil degradation due to the increased metal load, as well as low organic matter concentrations and acidic pH. However, such soils can be restored with the use of remediation techniques. Remediation can be achieved with the use of materials which have the capacity to retain metals through changes in soil characteristics, such as pH.

One method which has been widely used for soil remediation is the addition of crushed mussel shells (CMS). CMS is a by-product by the canning and seafood industry, which is normally removed and discarded, but has lately been used for soil remediation. Mussel shells comprise about 95% calcium carbonate and have a basic pH of 8.5 to 9.5 and thus have the ability to reduce metal toxicity through their capacity for heavy metal retention and pH increase. The aim of this study was to investigate the recovery of different soil types affected by metals against Cu and Zn during the time span of 2 weeks, as well as how remediation developed during time and its correlation with pH changes.

In order to perform the remediation, soils from mining and agricultural land were treated with three CMS doses; 0 g/kg, 24 g/kg and 48 g/kg CMS. They were then let to incubate for 14 days, and during that period, bacterial growth and pH were measured 5 times. On day 14, incubation was stopped and tolerance to Cu, Zn and the antibiotic tetracycline were measured for the determination of metal tolerance and antibiotic co-tolerance retention. Later, bacterial growth was measured after the pH had been adjusted with the addition of standard pH buffers, in order to establish whether the CMS addition had affected community pH traits.

Results
The pH of all soils showed an increase, even from the first day when the soils came in contact with the CMS. Highly contaminated mining soils exhibited the biggest change in pH, which by the end of the 14 days had risen for up to 2.2 pH points. This affected the bacterial growth rates, which by the end of the remediation period were higher in the treated soils than in the untreated. The test for tolerance against Cu, Zn and tetracycline showed increased tolerance in the soils that were pre-exposed to metals, yet with the treated samples being less tolerant compared to their untreated pair. This means that the soils which were treated with the 48 g/kg CMS dose were less tolerant to heavy metals than the soils treated with the 24 g/kg dose, and the soils treated with the 24 g/kg dose were less tolerant than the soils which were not treated (o g/kg CMS dose). These results came to prove that the remediation treatment can reduce tolerance against the contaminants tested. However, this tolerance pattern was not the same for tetracycline. Instead, the treated soils were more tolerant to the antibiotic rather than the untreated soils, especially in mining soils, whilst agricultural soils exhibited no pattern for antibiotic tolerance. This is contradictory to previous findings, which support correlated metal and antibiotic co-tolerance in soil microbiota. The results from the bacterial growth measurements in adjusted pH showed that the CMS treatment affected the pH traits of bacterial communities. The optimal pH values of bacterial community trait distribution were different between treatments. The bacterial pH trait distribution seemed to had followed the increases in pH, since the highest dose treated samples (46 g/kg CMS dose) showed higher optimal pH values. These results also showed that mining soils had wider ranges of optimal pH values, which indicates that these soils had suffered intense fluctuations in pH.

In conclusion, it was indicated that CMS is an effective method for treating heavy metal contaminated soils. The increases in soil pH due to the addition of CMS were correlated with reduced tolerance to the metals, since the CMS had an apparent effect on community pH-traits. This correlation is illustrated in the Figure. A similar correlation was not observed in agricultural soils. Even though this method was not as efficient for the removal of antibiotic toxicity, it was proven to be efficient in retaining metal availability and reducing their toxicity, thus allowing the bacteria to develop.

Master’s Degree Project in Biology, 30 credits, 2023
Department of Biology, Lund University

Advisor(s): Johannes Rousk, Vanesa Santas-Miguel
Microbial Ecology, Department of Biology, Lund University

• author: Alexandropoulou, Styliani
• supervisor: Johannes Rousk; Vanesa Santas
• organization: Degree Projects in Biology
• course: BIOM02 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9134352
• date added to LUP: 2023-08-15 14:20:46
• date last changed: 2023-08-15 14:20:46