Remediation of contaminated soil and groundwater using chemical reduction and solidification/stabilization method: a case study

Groundwater Cr(VI) contamination and remediation: A review from 1999 to 2022

Hexavalent chromium (Cr(VI)) contamination of groundwater has traditionally been an environmental issue of great concern due to its bioaccumulative and highly toxic nature. This paper presents a review and bibliometric analysis of the literature on the interest area "Cr(VI) in groundwater" published in the Web of Science Core Collection from 1999 to 2022. First, information on 203 actual Cr(VI)-contaminated groundwater sites around the world was summarized, and the basic characteristics of the sources and concentrations of contamination were derived. 68.95% of the sites were due to human causes and 56.43% of these sites had Cr(VI) concentrations in the range of 0-10 mg/L. At groundwater sites with high Cr(VI) contamination due to natural causes, 75.00% of the sites had Cr(VI) concentrations less than 0.2 mg/L. A total of 936 papers on "Cr(VI) in groundwater" were retrieved for bibliometric analysis: interest in research on Cr(VI) in groundwater has grown rapidly in recent years; 59.4% of the papers were published in the field of environmental sciences. A systematic review of the progress of studies on the Cr(VI) removal/remediation based on reduction, adsorption and biological processes is presented. Out of 666 papers on Cr(VI) removal/remediation, 512, 274, and 75 papers dealt with the topics of reduction, adsorption, and bioremediation, respectively. In addition, several studies have demonstrated the potential applicability of natural attenuation in the remediation of Cr(VI)-contaminated groundwater. This paper will help researchers to understand and investigate methodological strategies to remove Cr(VI) from groundwater in a more targeted and effective manner.

NaH2PO4 synergizes with organic matter to stabilize chromium in tannery sludge

Heavy metal stabilization is an effective method to treat chromium in tannery sludge. Here we show that mainly investigated NaH2PO4 (MSP) and organic matter (OM) to stabilize chromium in tannery sludge. The experimental investigation revealed that the addition of montmorillonite (MMT) and MSP samples showed a significant increase in the percentage of reducible and oxidizable Cr in the former compared to the samples with the addition of MMT. This is attributed to the formation of Cr-O bond, which allows the MSP to undergo an inner-sphere complexation reaction with the metal oxide of Cr via ligand exchange. Significantly, the MSP moiety adsorbs on the surface of OM through monodentate, which increases the adsorption sites of OM for Cr6+ and promotes the reduction of Cr6+ to Cr3+. Moreover, PO43- reacts with Cr3+ to produce CrPO4 precipitation, thus reducing the free Cr3+ content. Finally, DFT calculations confirmed that a ternary system is formed between PO43-, OM, and Cr, and the binding energy is negative, which indicated that PO43- could co-stabilize Cr with OM.

Simultaneous remediation of arsenic and organic chemicals contaminated soil and groundwater using chemical oxidation and precipitation/stabilization: a case study

After the departure of industrial facilities, reuse of the land in developed cities in China is problematic, due to the land contamination issues. The rapid remediation of sites with complex contamination is crucial and urgently needed. Herein, the case of on-site remediation of arsenic (As) in soil, as well as benzo(a)pyrene, total petroleum hydrocarbons, and As in groundwater was reported. For contaminated soil, the oxidant and deactivator (consisting of 20% sodium persulfate, 40% ferrous sulfate (FeSO4), and 40% portland cement) were applied to oxidize and immobilize As. As a result, the total amount and lixivium concentration of As were constrained under 20 mg/kg and 0.01 mg/L, respectively. Meanwhile, for contaminated groundwater, As and organic contaminants were treated by FeSO4/ozone and FeSO4/hydrogen peroxide with mass ratios of 1:5 and 1:8, respectively. The continuous monitoring of contaminants in 22 monitoring wells shown that all contaminants in groundwater were treated to meet the standards. In addition, the risk of secondary pollution and operation cost was effectively reduced by proper disposal and resourceful utilization. The findings indicated that the method of oxidation and precipitation/stabilization is technically, environmentally, and economically feasible for the remediation of contaminated sites with similar complex pollutants.

Effect of alkaline washing treatment on leaching behavior of municipal solid waste incineration bottom ash

This study aimed to find an effective, inexpensive, and safe washing treatment for municipal solid waste incineration bottom ash (MSWIBA) in order to reduce its potential harmful effects in disposal and recycling. The washing solutions, namely tap water (TW), saturated lime water (SLW), and wastewater from concrete batching plant (WW) were used to wash MSWIBA at different liquid-solid (L/S) ratios and for different durations. Leaching behavior of some heavy metals, chloride, and sulfate from MSWIBA was tested and evaluated. From the TCLP leaching test, when the L/S ratio was above 5, WW was the most effective solution in reducing As, Cd, Se, and Sb emissions from MSWIBA. The calcium and iron ions present in the WW were essential for controlling the leaching of As, Cd, and Sb from MSWIBA due to the formation of stable crystalline pharmacosiderite, cadmium hydroxide sulfate, and hydromeite during the washing process. Using WW showed the best effect in removing sulfate from MSWIBA. At a L/S ratio of 10, about 83% of the sulfate could be removed from MSWIBA after 20 min of washing. The L/S ratio was most influential in removing chloride from MSWIBA. The three washing treatments chosen were effective in reducing the chloride level in MSWIBA to below the level of hazardous waste. Nevertheless, there were still substantial amounts of chloride remaining in the treated MSWIBA. Under the Dutch Building Materials Decree, the treated MSWIBA may be used as a building material in parts which allow isolation, control, and monitoring (ICM).

Investigating the hydration of C3A in the presence of the potentially toxic element chromium-a route to remediation?

Pollution by hexavalent chromium is a growing, global problem. Its presence in public water systems is often the result of industrial activities, both past and present. In this study, tricalcium aluminate (C3A, Ca₃Al₂O₆) is added to solutions of varying concentrations of potassium chromate (K₂CrO₄) and samples of both the solid and liquid are taken at various time intervals to monitor the removal of chromium from the solutions. Solution concentrations of 0.2 M, 0.1 M, 0.02 M, and 0.01 M are used, and the chromium concentration is found to reduce in all cases. For the 0.02 M solution the chromium concentration is reduced from 1040 ppm to 3.1 ppm in 1 week, and the chromium concentration of the 0.01 M solution is reduced from 520 ppm to 0.26 ppm in only one day of reaction with the C3A. The chromium removed from solution is identified in the solid products, which were fully characterised as being a mixture of ettringite (Ca₆[Al(OH)₆]₂(CrO₄)₃·26H₂O) and monochromate (Ca₄[Al(OH)₆]₂CrO₄·8H₂O) phases from analysis of Powder X-ray Diffraction and Fourier Transform Infrared Spectroscopy data. The work presented here is a proof of concept study to investigate C3A as a potential material for the removal of hexavalent chromium from solution. The results from this study are initial steps towards development of this as a technology for hexavalent chromium remediation.

A calcium aluminate material has been successfully used to remove toxic hexavalent chromium from water. A thorough powder X-ray diffraction study was done to determine the composition of the products.

Combined Treatment of Cr(VI)-Contaminated Soils by Reduction, Adsorption, and Solidification

Remediation of Cr(VI)-contaminated soil usually includes reducing Cr(VI) to Cr(III) with sub-sequent solidification. In this paper, a treatment technique that combines reduction, adsorption, and solidification was proposed. By introducing an adsorbent into the reduction process, the remediation effectiveness was improved and the amount of reducing and solidified agent was decreased. Synthetic precipitation leaching procedure (SPLP), unconfined compressive strength (UCS) test, and scanning electron microscope (SEM) analysis were carried out to evaluate the remediation effect under different agent combinations and different agent-adding procedures. The results of SPLP showed that the reduction/adsorption/solidification treatment significantly reduced the leachability of Cr. UCS increased with increasing dosage of cement and CaS5, and decreased with an increasing dosage of vermiculite. The best agent dosage was CaS5 of 2 times molar stoichiometric ratio of Cr(VI), 15% of vermiculite, and 20% of cement. Orthogonal test showed that for soil with low Cr(VI) content, CaS5 dosage was the most important factor that affected the leachability of Cr. Cement and vermiculite have greater impact in limiting the leachability of Cr when Cr(VI) content in soil increased.

Solidification, remediation and long-term stability of heavy metal contaminated soil under the background of sustainable development

At present, the global pollution has seriously exceeded the standard. With the passage of time, pollution has gradually affected people's daily lives, but the solution to pollution is far from achieving a better treatment effect. For the treatment of pollution, in addition to considering the treatment effect, it is also necessary to consider whether the treatment method will cause pollution and the cost of the treatment of the pollutants. As one of the lifelines of human survival, the land is also suffering from pollution. The impact of heavy metal pollution is particularly serious, and there is no better solution. Based on this, this paper proposes a curing agent based on sustainable remediation to solve the soil pollution of heavy metals. The main material is Basic oxygen furnace slag (BOFS), which has excellent social development characteristics in all aspects, and the raw materials are calcium carbide residue (CCR) and phosphogypsum (PG) to explore a more suitable curing agent. (consisting of BOFS, CCR, and PG, abbreviated as BCP). The experimental results in this paper show that the volume of pores and pores in the agglomerates are slightly reduced, and the content of curing agent is increased from 4 to 10%, while the corresponding volume is only reduced by 0.006 and 0.017 mL/g. Therefore, it can be seen that the reduction of the pore volume between the aggregates of the stabilized species of BCP has made a major contribution to the strength development.

Stabilization and remediation of heavy metal-contaminated soils in China: insights from a decade-long national survey

Recently, enterprises and industries associated with intense pollution have been relocated in China, leaving behind abandoned polluted sites. Consequently, stabilization has attracted unprecedented attention and rapid development. However, too much focus has been placed on practicality and short-term effectiveness, whereas long-term effectiveness and sustainability concerns have been overlooked. The present study reports the findings of a national survey administered to stakeholders involved in soil stabilization projects over a decade to determine the pollution characteristics of industrial sites and current utilization status of stabilization technologies in China. One-hundred and fifty soil stabilization projects surveyed revealed that among 29% of chemical industry remained sites, 96.7% of the sites were heavily polluted with heavy metals and metalloids, mainly in the forms of lead, arsenic, and chromium. Our analyses of soil reuse methods revealed that landfilling was the primary strategy of soil disposal following stabilization and remediation. In addition, the reuse of treated soils is preferred to landfill waste treatment. Therefore, from the perspective of economy and reducing landfill loads, environmental management measures should be adopted based on the development objectives of different regions to avoid the failure of stabilization treatments and reapplication and over-repair problems, with the aim of establishing an evaluation method of "site-specific, analysis-specific" evaluation method.