Optimization of Cadmium Removal from Aqueous Solutions Using Walnut-shell Residues Biochar Supported/unsupported by Nanoscale Zero-valent Iron through Response Surface Methodology

Using various biochars to remove heavy metals (HMs) from aqueous solutions has been increased in recent years. It is believed that the use of nanocompounds in biochars surface structure may increase the efficiency of contaminants removal. Therefore, this research tries to investigate the efficiency of walnut-shell biochar (WSB) alone or supported by nanoscale zero-valent iron (WSB-nZVI) on cadmium (Cd) removal in aqueous solution controlled by four variables including initial Cd concentration, initial solution pH, contact time, and adsorbent dosage by Box Behnken design under response surface methodology. The results of present study showed that WSB-nZVI has a significant priority on WSB of Cd removal efficiency in aqueous solutions. The existence of functional groups on the surface of WSB via precipitation and adsorption processes, as well as nZVI formed on the WSB-nZVI via generating adsorption and complexation processes, have increased the ability Cd removal than WSB raw adsorbent. The maximum predicted Cd removal efficiency based on the proposed model was 99.72% with desirability of 1, in initial Cd concentration of 70.78 mg L-1, pH of 6.92, adsorbent dose of 0.56 g L-1 and contact time of 40.42 min

Assessment of content and chemical forms of arsenic, copper, lead, and chromium in sewage sludge compost as affected by various bulking agents

In current study, the effect of various organic substances as bulking agents (BAs) including wheat straw, pistachio hull wastes, and tree leaves at different levels (10, 25, 45% v/v) were investigated on total concentration and chemical forms of Cu, Pb, Cr, and As in sewage sludge (SS) compost prepared by windrow method. According to the results, the composting process (with/without BAs), due to losses of SS mass and volume, increased the total concentration of heavy metals (HMs) compared to the un-composted SS sample (RSS). Evaluation of HMs chemical forms in prepared compost sample without BAs application (CSS) showed that the composting process reduced the mobility factor of As (from 28% to 20%), Pb (from 11.6% to 9.3%), and Cr (from 14.5% to 9.2%) compared to the RSS. Application of three BAs considerably decreased the mobility factor of As (17.5-18.8%), Pb (4.8-7.9%), and Cr (1.4-6.8%) compared to CSS and RSS. Changes of Cu mobility in prepared compost samples showed an unclear trend, however in some treatments, due to transferred organic fraction into exchangeable and carbonate fractions, increasing of this factor was obvious. Generally, the composting appeared to reduce As, Pb, and Cr availability by stabilizing the three metals and making them more stable and less mobile. In addition, the BAs application effect on HMs behavior of SS compost samples were so different and no specific kind of BAs can be recommended as a superior BAs in SS composting process

Immobilization of Cadmium in a Cd-Spiked Soil by Different Kinds of Amendments

   Chemical stabilization of heavy metals is one of the soil remediation methods based on the application amendments to reduce mobility of heavy metals. A laboratory study was conducted to investigate the influence of different kinds of amendments on cadmium (Cd) stabilization in a Cd-spiked soil. The amendments were municipal solid waste compost (MSWC), Coal fly ash (CFA), rice husk biochars prepared at 300°C (B300) and 600°C (B600), zero valent iron (Fe0) and zero valent manganese (Mn0). The Cd-spiked soils were separately incubated with selected amendments at the rates of 2 and 5% (W/W) for 90 days at 25 °C. Soil samples were extracted by EDTA for periods of 5 to 975min. In addition, sequential extraction was used as a suitable method for identification of chemical forms of Cd and their plant availability. The addition of amendments to soil had significant effects on desorption and chemical forms of Cd. Changes in Cd fractions and their conversion into less soluble forms were clear in all treated soils. The addition of amendments resulted in a significant reduction in mobility factor of Cd compared to the control treatment. Among all amendments tested, Fe0 was the most effective treatment in decreasing dynamic of Cd. Biphasic pattern of Cd desorption kinetic was fitted well by the model of two first-order reactions. In general, from the practical point of view, Fe0, MSWC and Mn0 treatments are effective in Cd immobilization, while application of  Fe0 at 5% (W/W) was the best treatment for stabilization of Cd. </div