Effects of the joint application of phosphate rock, ferric nitrate and plant ash on the immobility of As, Pb and Cd in soils

Highlights

• Plant ash helped on phosphate rock and ferric salts to immobilize soil heavy metal(loid)s.

• The application of plant ash clearly reversed the mobility of arsenic (As) in soils.

• Plant ash contributed to further immobility of lead (Pb) and cadmium (Cd) in soils.

• There was insignificant change of the soil pH by the plant ash.

• Multiply characterizations on the soil-metal(loid) interaction have been addressed.

Abstract

Phosphate rock (PR) and ferric salts have been frequently used to immobilize heavy metal(loid)s in soils, but in varied efficiencies referring to different metal(loid) pollutants. This study explored the effective application of plant ash (PA) to the previous formula of phosphate rock (PR) and ferric salts (Fe(NO₃)₃) (PR + Fe³⁺+PA), compared to only PR, on the bioavailability and immobility of multi-metal(loid)s of selected arsenic (As), lead (Pb) and cadmium (Cd) in soils. Results from NaHCO₃- extraction and toxicity characteristic leaching procedure (TCLP) implied the increase of the As mobility in soils by 7.0% and 2.6% using PR only, but the significant reduction of the As mobility by 24.2% and 82.4% jointly using PR + Fe³⁺+PA. Meanwhile, the application of either PR alone or PR + Fe³⁺+PA in soil significantly decreased Pb and Cd extracting in diethylene triamine pentacetate acid (DTPA) and TCLP, particularly, the immobilization effect of PR + Fe³⁺+PA was better than that of PR. The leaching column test further confirmed the high durability of PR + Fe³⁺+PA on the immobilization of As and Pb under the continuous acid exposure, but likely slightly increased the mobility of Cd (the accumulated concentration of Cd, 5.88 μg/L) compared to that (3.16 μg/L) in the untreated column (UN-column), which were both much lower than the level V (100 μg/L) of the Chinese National Quality Standard for Surface Water (GB 3838–2002). Therefore, PR + Fe³⁺+PA exhibited the significant enhancement on the immobilization of As, Pb and Cd under simulated acid rain (SAR) leaching.

Introduction

Heavy metal(loid) pollution in soils are serious problems on environment and human life. Among of them, arsenic (As), lead (Pb) and cadmium (Cd) have attracted special attention due to their pollution abundance, toxicity and phytotoxicity (Wei et al., 2015). This is especially true in China. According to the national soil pollution investigation report issued by two ministries of the Chinese government on April 17, 2014, Cd has been officially identified as the most important inorganic pollutant in Chinese soil, while As and Pb are also listed as two of the eight inorganic pollutants (Yin et al., 2016). Mining, smelting and other human activities seriously pollute the surrounding environment and farmland. For example, As, Pb and Cd in farmland along Diaojiang river in Guangxi Province are seriously over standard due to the casual discharge of tailings in the mining area, and their contents are as high as 88.23–12143.75 mg/kg, 80.18–1539.27 mg/kg and 3.01–99.96 mg/kg (Song et al., 2003). The concerned metal(loid)s in soil can be transferred to plants and then into the food chain, resulting in phytotoxicity and threat to human health.

The remediation of metal-contaminated soils can be expensive and arduous. Available methods involve mechanical separation, chemical cleaning, electrical remediation and immobilization(Lianwen et al., 2018). Among them, the in-situ chemical immobilization of heavy metal(loid)s in contaminated soil is considered economic- and environmental-sound. It usually involves the use of soil amendments, such as organic waste, compost (CP) and phosphate compounds, leading effects such as adsorption, surface precipitation, formation of stable organic ligands complexes and ion exchange of concerned metal(loid)s (Bolan et al., 2003). Meanwhile, the soil pH is very important on the immobility of metal(loid)s (As, Pb and Cd). Chen (Jing et al., 2004) found that the acid environment (pH = 4–6.5) facilitated the As adsorption to soils, and the alkaline environment (pH = 7–8.5) released As from soils. Yin (Liping et al., 2014) pointed out that both strong acid and strong alkaline conditions were conducive to the Pb release from soils, while the weak acid or weak alkaline promoted Pb existed with soils in the form of residue and iron manganese oxidation. The increase of the soil pH decreased the Cd solubility and enhanced its adsorption on the soil colloid (Liping et al., 2014).

Many heavy metal(loid)s can form stable phosphate precipitation under various environmental conditions. This inspired applications of many natural or synthetic phosphor materials for immobilizing heavy metal(loid)s in soils, including apatite, phosphate rock, phosphoric acid or phosphate (Kumpiene et al., 2008). Phosphate rock (PR), as a direct, economic and durable source of phosphorus (Fayiga and Ma, 2006), has been tried in soils in recent years to replace chemical substances (Husnain et al., 2014). Compared to the synthesized phosphate fertilizers, PR is lower in its cost, longer in its residual effects, and thus effective in its utilization. The disadvantage of PR is its low solubility, and thus not conducive to its direct use as a source of phosphorus for soil amendments. This can be somehow cancelled by the unavoidable acid rain leaching. On the other hand, there were reports on the negative impact of phosphate on As. Phosphate may replace or inhibit arsenate adsorption, resulting in As activation (Antelo et al., 2015a). There is the likelihood of the occurrence of the As mobilization in co-contaminated (As, Pb and Cd) soils when whatever phosphor materials are applied.

Metal oxides can immobile As in soils through mechanisms of adsorption/desorption and co-precipitation (Jiang et al., 2017). The iron matrix, such as iron salts and iron oxides, is commonly used to immobilize As in soils and inhibit the As intake by plants. Usually iron (III) salts are more effective than iron (II) salts (Xenidis et al., 2010a). Some scholars have used zero-valent iron and iron oxides as soil amendments for As contaminated soil, but found they are less effective than iron salts. The dosage of zero-valent iron, on the basis of the iron content, was three times as much as that of iron salt (Tiberg et al., 2016). However, the zero-valent iron in soils may self-form a fixed oxidation product layer on its surface, hindering the further reaction of zero-valent iron (Cheyns et al., 2012). The iron salts can cause soil acidification via its hydrolysate, therefore, it is necessary to add a little alkaline auxiliary agent to avoid the soil acidification while iron salts applying.

Plant ash (PA) is the residue of burnt plants, containing alkaline minerals from plants, which can effectively prevent soil acidizing. On the other hand, PA contains macro and micro nutrients for the plant growth, such as calcium (Ca), potassium (K), magnesium (Mg), aluminum (Al), iron (Fe) and some trace elements (Park et al., 2005). Etiegni, L. (Etiegni et al., 2008) found lower than 2% of PA was not harmful to the growth plants (winter wheat and poplar); Nabeela, F. (Nabeela et al., 2015) revealed that PA can promote germination and growth of seeds below 25 g/kg but inhibit them above 100 g/kg. Therefore, the proper application of PA in soils can help to plants, preventing the environment damage and is likely to promote the fixation of heavy toxic metal(loid)s because of its higher specific surface area and larger pore volume. However, this potential was so far not fully understood.

The chemical behaviors of As, Pb and Cd in soils are quite different. It seemed difficult to use a single amendment achieving synergistic and simultaneous immobility of wide coverage of heavy metal(loid)s, especially As, Pb and Cd. Therefore, the combination of various amendments will be potentially required in the remediation of heavy metal contaminated soils. This study explored a strategy to use a joint formula, involving PR, ferric nitrate (Fe³⁺) and PA as amendments. The evaluation of the leachability and immobilization of As, Pb and Cd in contaminated soils, via approaches of (1) the determination of extractable As, Pb and Cd using methods of DTPA-, NaHCO₃- and TCLP-, (2) the different components of As, Pb and Cd by two sequential extraction procedures and (3) the leachability of As, Pb and Cd in a column facility.